diff --git a/vllm/model_executor/models/mistral3.py b/vllm/model_executor/models/mistral3.py
index 94cbb20cee5f..f8e9e3181367 100644
--- a/vllm/model_executor/models/mistral3.py
+++ b/vllm/model_executor/models/mistral3.py
@@ -272,6 +272,9 @@ class Mistral3MultiModalProcessor(
 
         vision_config = hf_config.vision_config
         assert isinstance(vision_config, PixtralVisionConfig)
+        # Need to sneak in spatial_merge_size for Mistral3
+        vision_config.spatial_merge_size = getattr(hf_config,
+                                                   "spatial_merge_size", 1)
         encoder_info = PixtralHFEncoderInfo(vision_config)
 
         def get_replacement(item_idx: int):
diff --git a/vllm/model_executor/models/pixtral.py b/vllm/model_executor/models/pixtral.py
index 58323d639d5d..73fd80146955 100644
--- a/vllm/model_executor/models/pixtral.py
+++ b/vllm/model_executor/models/pixtral.py
@@ -911,9 +911,8 @@ class PixtralHFEncoderInfo(VisionEncoderInfo[PixtralVisionConfig]):
         return self.vision_config.image_size
 
     def get_patch_size(self) -> int:
-        spatial_merge_size = getattr(self.vision_config, "spatial_merge_size",
-                                     1)
-        return (self.vision_config.patch_size * spatial_merge_size)
+        return (self.vision_config.patch_size *
+                self.vision_config.spatial_merge_size)
 
     def get_patch_grid_length(self) -> int:
         image_size, patch_size = self.get_image_size(), self.get_patch_size()