diff --git a/tests/reasoning/test_olmo3_reasoning_parser.py b/tests/reasoning/test_olmo3_reasoning_parser.py
new file mode 100644
index 0000000000000..067be1ea29966
--- /dev/null
+++ b/tests/reasoning/test_olmo3_reasoning_parser.py
@@ -0,0 +1,157 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+from transformers import AutoTokenizer
+
+from tests.reasoning.utils import run_reasoning_extraction
+from vllm.reasoning import ReasoningParser, ReasoningParserManager
+
+parser_name = "olmo3"
+START_REASONING = "<think>"
+END_REASONING = "</think>"
+
+NO_REASONING = {
+    "output": f"{START_REASONING}{END_REASONING}No thoughts, head empty!",
+    "reasoning_content": None,
+    "content": "No thoughts, head empty!",
+}
+
+NO_REASONING_WITH_NEWLINE = {
+    "output":
+    f"{START_REASONING}\n{END_REASONING}\n\nNo thoughts, head empty!",
+    "reasoning_content": "\n",
+    "content": "\n\nNo thoughts, head empty!",
+}
+
+SIMPLE_REASONING = {
+    "output":
+    f"{START_REASONING}This is a reasoning section{END_REASONING}This is the rest",  # noqa: E501
+    "reasoning_content": "This is a reasoning section",
+    "content": "This is the rest",
+}
+
+SIMPLE_REASONING_WITH_NEWLINE = {
+    "output":
+    f"{START_REASONING} Look!\n\nI'm thinking...{END_REASONING}\nThis is the rest",  # noqa: E501
+    "reasoning_content": " Look!\n\nI'm thinking...",
+    "content": "\nThis is the rest",
+}
+
+SIMPLE_REASONING_WITH_MULTIPLE_NEWLINES = {
+    "output":
+    f"{START_REASONING}\nLook!\nI'm thinking...\n\n{END_REASONING}\n\n\nThis is the rest",  # noqa: E501
+    "reasoning_content": "\nLook!\nI'm thinking...\n\n",
+    "content": "\n\n\nThis is the rest",
+}
+
+NO_REASONING_ONLY_END_THINK = {
+    "output": f"{END_REASONING}\n\nNo thoughts, head empty!",
+    "reasoning_content": None,
+    "content": "\n\nNo thoughts, head empty!",
+}
+
+REASONING_ONLY_END_THINK = {
+    "output":
+    f"The user is asking me not to think.{END_REASONING}No thoughts!",
+    "reasoning_content": "The user is asking me not to think.",
+    "content": "No thoughts!",
+}
+
+TEST_CASES = [
+    pytest.param(
+        False,  # not streaming
+        NO_REASONING,
+        id="no_reasoning",
+    ),
+    pytest.param(
+        False,  # not streaming
+        NO_REASONING_WITH_NEWLINE,
+        id="no_reasoning_with_newline",
+    ),
+    pytest.param(
+        False,  # not streaming
+        SIMPLE_REASONING,
+        id="simple_reasoning",
+    ),
+    pytest.param(
+        False,  # not streaming
+        SIMPLE_REASONING_WITH_NEWLINE,
+        id="simple_reasoning_with_newline",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        SIMPLE_REASONING_WITH_MULTIPLE_NEWLINES,
+        id="simple_reasoning_with_multiple_newlines",
+    ),
+    pytest.param(
+        False,  # not streaming
+        NO_REASONING_ONLY_END_THINK,
+        id="no_reasoning_only_end_think",
+    ),
+    pytest.param(
+        False,  # not streaming
+        REASONING_ONLY_END_THINK,
+        id="yes_reasoning_only_end_think",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        NO_REASONING,
+        id="no_reasoning_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        NO_REASONING_WITH_NEWLINE,
+        id="no_reasoning_with_newline_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        SIMPLE_REASONING,
+        id="simple_reasoning_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        SIMPLE_REASONING_WITH_NEWLINE,
+        id="simple_reasoning_with_newline_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        SIMPLE_REASONING_WITH_MULTIPLE_NEWLINES,
+        id="simple_reasoning_with_multiple_newlines_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        NO_REASONING_ONLY_END_THINK,
+        id="no_reasoning_only_end_think_streaming",
+    ),
+    pytest.param(
+        True,  # enable streaming
+        REASONING_ONLY_END_THINK,
+        id="yes_reasoning_only_end_think_streaming",
+    ),
+]
+
+# Global tokenizer initialization to avoid repeated loading
+tokenizer = AutoTokenizer.from_pretrained("allenai/dolma2-tokenizer")
+
+
+@pytest.mark.parametrize("streaming, param_dict", TEST_CASES)
+def test_reasoning(
+    streaming: bool,
+    param_dict: dict[str, str],
+):
+    output = tokenizer.tokenize(param_dict["output"])
+
+    # decode everything to tokens
+    model_output: list[str] = [
+        tokenizer.convert_tokens_to_string([token]) for token in output
+    ]
+    parser_cls = ReasoningParserManager.get_reasoning_parser(parser_name)
+    parser: ReasoningParser = parser_cls(tokenizer)
+
+    reasoning, content = run_reasoning_extraction(reasoning_parser=parser,
+                                                  model_output=model_output,
+                                                  streaming=streaming)
+
+    assert reasoning == param_dict["reasoning_content"]
+    assert content == param_dict["content"]
diff --git a/vllm/reasoning/__init__.py b/vllm/reasoning/__init__.py
index 3c8a9c6ae0d33..78d3bf35f2a32 100644
--- a/vllm/reasoning/__init__.py
+++ b/vllm/reasoning/__init__.py
@@ -9,6 +9,7 @@ from .gptoss_reasoning_parser import GptOssReasoningParser
 from .granite_reasoning_parser import GraniteReasoningParser
 from .hunyuan_a13b_reasoning_parser import HunyuanA13BReasoningParser
 from .mistral_reasoning_parser import MistralReasoningParser
+from .olmo3_reasoning_parser import Olmo3ReasoningParser
 from .qwen3_reasoning_parser import Qwen3ReasoningParser
 from .seedoss_reasoning_parser import SeedOSSReasoningParser
 from .step3_reasoning_parser import Step3ReasoningParser
@@ -23,6 +24,7 @@ __all__ = [
     "Qwen3ReasoningParser",
     "Glm4MoeModelReasoningParser",
     "MistralReasoningParser",
+    "Olmo3ReasoningParser",
     "Step3ReasoningParser",
     "GptOssReasoningParser",
     "SeedOSSReasoningParser",
diff --git a/vllm/reasoning/olmo3_reasoning_parser.py b/vllm/reasoning/olmo3_reasoning_parser.py
new file mode 100644
index 0000000000000..e37b6d887515f
--- /dev/null
+++ b/vllm/reasoning/olmo3_reasoning_parser.py
@@ -0,0 +1,294 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import dataclasses as dt
+import enum
+from collections.abc import Sequence
+from typing import TYPE_CHECKING, Optional, Union
+
+import regex as re
+
+if TYPE_CHECKING:
+    from vllm.transformers_utils.tokenizer import AnyTokenizer
+
+from vllm.entrypoints.openai.protocol import (ChatCompletionRequest,
+                                              DeltaMessage, ResponsesRequest)
+from vllm.logger import init_logger
+from vllm.reasoning import ReasoningParser, ReasoningParserManager
+
+logger = init_logger(__name__)
+
+
+class Olmo3ReasoningState(enum.Enum):
+    REASONING = 1
+    CONTENT = 2
+
+
+@dt.dataclass(frozen=True)
+class Indices:
+    start: int
+    end: int
+
+    def __len__(self):
+        return self.end - self.start
+
+
+def string_overlap(a: str,
+                   b: str) -> tuple[Optional[Indices], Optional[Indices]]:
+    """
+    Find the longest overlap where the end of string a matches the start
+    of string b.
+
+    Args:
+        a: First string
+        b: Second string
+
+    Returns:
+        Tuple of IndicesTuples representing the overlapping portions in each
+        string, or a tuple of None if no overlap exists
+    """
+
+    # swap so a is always the shorter string
+    a, b, swap = (a, b, False) if len(a) < len(b) else (b, a, True)
+
+    # first check: is a fully contained in b?
+    if a in b:
+        ind_a = Indices(0, len(a))
+        ind_b = Indices(b.index(a), b.index(a) + len(a))
+        return (ind_b, ind_a) if swap else (ind_a, ind_b)
+
+    # second check: does the end of a overlap with the
+    #               beginning of b?
+    for i in range(len(a) - 1, 0, -1):
+        if a[-i:] == b[:i]:
+            ind_a = Indices(len(a) - i, len(a))
+            ind_b = Indices(0, i)
+            return (ind_b, ind_a) if swap else (ind_a, ind_b)
+
+    # third check: does the beginning of a overlap with
+    #              the end of b?
+    for i in range(len(a) - 1, 0, -1):
+        if b[-i:] == a[:i]:
+            ind_a = Indices(0, i)
+            ind_b = Indices(len(b) - i, len(b))
+            return (ind_b, ind_a) if swap else (ind_a, ind_b)
+
+    return None, None
+
+
+@dt.dataclass
+class Olmo3ReasoningBuffer:
+    think_start: str = "<think>"
+    think_end: str = "</think>"
+    buffer: str = ""
+
+    # we start in reasoning state to support cases where we hardcode
+    # <think> as the start of the reasoning block.
+    # In those cases, the only token we will see is </think>, which
+    # is when we switch to content state.
+    state: Olmo3ReasoningState = Olmo3ReasoningState.REASONING
+
+    def process_buffer(self) -> Optional[DeltaMessage]:
+        start_think_idx = self.buffer.find(self.think_start)
+
+        if start_think_idx >= 0:
+            self.state = Olmo3ReasoningState.REASONING
+            pretext, self.buffer = (
+                self.buffer[:start_think_idx],
+                self.buffer[start_think_idx + len(self.think_start):],
+            )
+            if start_think_idx > 0:
+                # this covers the case there's content before
+                # the start of the reasoning block
+                return DeltaMessage(content=pretext)
+
+        end_think_idx = self.buffer.rfind(self.think_end)
+
+        if end_think_idx >= 0:
+            self.state = Olmo3ReasoningState.CONTENT
+            pretext, self.buffer = (
+                self.buffer[:end_think_idx],
+                self.buffer[end_think_idx + len(self.think_end):],
+            )
+            if end_think_idx > 0:
+                # this covers the case there's content before
+                # the end of the reasoning block
+                return DeltaMessage(reasoning_content=pretext)
+
+        if self.state == Olmo3ReasoningState.REASONING:
+            # we are inside reasoning block, return and empty
+            # the text buffer
+            (
+                text_buffer,
+                self.buffer,
+            ) = self.buffer, ""
+            return DeltaMessage(reasoning_content=text_buffer)
+
+        if self.state == Olmo3ReasoningState.CONTENT:
+            # we are outside reasoning block, return and empty
+            # the text buffer
+            (
+                text_buffer,
+                self.buffer,
+            ) = self.buffer, ""
+            return DeltaMessage(content=text_buffer)
+
+        # nothing to return unless we are in reasoning or content state
+        return None
+
+    def __len__(self):
+        # is the length of the text buffer
+        return len(self.buffer)
+
+    def add_text(self, delta_text: str) -> Optional[DeltaMessage]:
+        # we start by adding the delta text to the buffer
+        self.buffer += delta_text
+
+        # setting this to empty before starting
+        delta_message: Optional[DeltaMessage] = None
+
+        # we start by computing the overlap between the delta_text
+        # and start/end of think tokens.
+        _, overlap_think_start = string_overlap(delta_text, self.think_start)
+        _, overlap_think_end = string_overlap(delta_text, self.think_end)
+
+        partial_overlap_start = overlap_think_start is not None and len(
+            overlap_think_start) < len(self.think_start)
+        partial_overlap_end = overlap_think_end is not None and len(
+            overlap_think_end) < len(self.think_end)
+
+        if (partial_overlap_start and self.think_start in self.buffer
+                and not partial_overlap_end):
+            # we can only process the buffer if partial overlap
+            # is the last part of think token (thus causing
+            # text_buffer to contain the start of think token)
+            # and there are no partial overlaps with end think
+            delta_message = self.process_buffer()
+
+        elif partial_overlap_end and self.think_end in self.buffer:
+            # same as before (partial overlap only allowed)
+            # if the buffer contains the end think token,
+            # but we don't have to check for partial overlap
+            # with start think token because they are handled
+            # by the previous condition
+            delta_message = self.process_buffer()
+
+        elif partial_overlap_start or partial_overlap_end:
+            # in general, if there are overlaps, we don't
+            # process the buffer because we want to wait until
+            # the think token is fully completed.
+            return None
+        else:
+            # we process the buffer as normal
+            delta_message = self.process_buffer()
+
+        return delta_message
+
+
+@ReasoningParserManager.register_module("olmo3")
+class Olmo3ReasoningParser(ReasoningParser):
+    """
+    Reasoning parser for Olmo 3 model
+
+    Olmo3ReasoningParser
+
+    This class implements a reasoning parser specifically designed for the
+    Olmo 3 family of models. Olmo 3 models do not use special tokens to
+    indicate reasoning; rather, reasoning trace is wrapped in `<think>` and
+    `</think>`, which are tokenized using standard vocabulary entries.
+    Because of this, the parser operates in string space, accumulating the
+    characters in a buffer until it sees `<think>` or `</think>`. tokens
+    to switch modes.
+
+    Key Features:
+        - For non-stream output, Recognizes and extracts reasoning (text
+          bracketed by `<think>` and `</think>`) and content (everything
+          after the first `</think>`).
+        - For stream process, it uses a buffer to accumulate delta text,
+          and output progressive delta messages as soon as thinking starts
+          or ends.
+        - For reliability, some Olmo 3 models may hardcode the first
+          `<think>` token is the input text (similar to Deepseek R1,
+          or reasoning-only Qwen models). To support such variants, the
+          parser can optionally work in cases where the first `<think>`
+          token is missing from generation.
+    """
+
+    def __init__(self, tokenizer: "AnyTokenizer", *args, **kwargs):
+        super().__init__(tokenizer, *args, **kwargs)
+
+        self.think_start = r"<think>"
+        self.think_end = r"</think>"
+
+        # notice that the first think is optional; this allows template to
+        # work in cases when we hardcode a <think> at the beginning of the
+        # reasoning template.
+        reasoning_expr = (rf"^(?:{self.think_start})?(?P<reasoning>.*?)" +
+                          rf"{self.think_end}(?P<content>.*)$")
+        self.reasoning_regex = re.compile(reasoning_expr, re.DOTALL)
+
+        self.buffer = Olmo3ReasoningBuffer(think_start=self.think_start,
+                                           think_end=self.think_end)
+
+    def is_reasoning_end(self, input_ids: list[int]) -> bool:
+        text = self.model_tokenizer.decode(input_ids)
+        return self.think_end in text
+
+    def extract_content_ids(self, input_ids: list[int]) -> list[int]:
+        # for Olmo 3 streaming reason parsing, the stream parse
+        # will call first, and the same token will be called in
+        # is_reasoning_end and extract_content_ids
+        # this id is not part of content, so just return [] here.
+        return []
+
+    def extract_reasoning_content(
+        self,
+        model_output: str,
+        request: Union[ChatCompletionRequest, ResponsesRequest],
+    ) -> tuple[Optional[str], Optional[str]]:
+        """Extract the reasoning content & content sections, respectively.
+        If the sequence doesn't match what we expect, i.e., the model generates
+        something else, all content is considered non-reasoning content.
+
+        Args:
+            model_output (str): Output of the model to be parsed.
+            request (ChatCompletionRequest | ResponsesRequest): Request being
+                processed.
+
+        Returns:
+            tuple[Optional[str], Optional[str]]: Tuple pair containing the
+            reasoning content and non-reasoning content.
+        """
+
+        re_match = self.reasoning_regex.match(model_output)
+        if re_match:
+            reasoning_content = re_match.group("reasoning") or None
+            content = re_match.group("content") or None
+            return reasoning_content, content
+
+        # no reasoning content
+        return None, model_output
+
+    def extract_reasoning_content_streaming(
+        self,
+        previous_text: str,
+        current_text: str,
+        delta_text: str,
+        previous_token_ids: Sequence[int],
+        current_token_ids: Sequence[int],
+        delta_token_ids: Sequence[int],
+    ) -> Union[DeltaMessage, None]:
+        """Extract content using token ID sequence state machine"""
+
+        delta_message = self.buffer.add_text(delta_text)
+        if (delta_message is None
+                and self.buffer.think_end in self.buffer.buffer):
+            # this is a bit hacky, but, because of how the buffer is
+            # constructed, if the last delta_text contains characters that
+            # marks the end of thinking tokens, then messages in the buffer
+            # would never be processed because we get no other turn. To get
+            # around that, we check if the text buffer contains the end of
+            # thinking tokens, and, if so, we reprocess the buffer again.
+            delta_message = self.buffer.process_buffer()
+
+        return delta_message