diff --git a/src/strands/models/anthropic.py b/src/strands/models/anthropic.py
index 812171a0c..c80b6b29a 100644
--- a/src/strands/models/anthropic.py
+++ b/src/strands/models/anthropic.py
@@ -28,6 +28,14 @@
 
 T = TypeVar("T", bound=BaseModel)
 
+IMAGE_MEDIA_TYPES = {
+    "gif": "image/gif",
+    "jpeg": "image/jpeg",
+    "jpg": "image/jpeg",
+    "png": "image/png",
+    "webp": "image/webp",
+}
+
 
 class AnthropicModel(Model):
     """Anthropic model provider implementation."""
@@ -131,10 +139,14 @@ def _format_request_message_content(self, content: ContentBlock) -> dict[str, An
             }
 
         if "image" in content:
+            image_format = content["image"]["format"]
             return {
                 "source": {
                     "data": base64.b64encode(content["image"]["source"]["bytes"]).decode("utf-8"),
-                    "media_type": mimetypes.types_map.get(f".{content['image']['format']}", "application/octet-stream"),
+                    "media_type": IMAGE_MEDIA_TYPES.get(
+                        image_format,
+                        mimetypes.types_map.get(f".{image_format}", "application/octet-stream"),
+                    ),
                     "type": "base64",
                 },
                 "type": "image",
diff --git a/tests/strands/models/test_anthropic.py b/tests/strands/models/test_anthropic.py
index 0ebdb161c..7366dc3c9 100644
--- a/tests/strands/models/test_anthropic.py
+++ b/tests/strands/models/test_anthropic.py
@@ -1,4 +1,5 @@
 import logging
+import mimetypes
 import unittest.mock
 import warnings
 
@@ -254,6 +255,21 @@ def test_format_request_with_image(model, model_id, max_tokens):
     assert tru_request == exp_request
 
 
+def test_format_request_with_webp_image_does_not_depend_on_mimetypes(model, model_id, max_tokens, monkeypatch):
+    monkeypatch.delitem(mimetypes.types_map, ".webp", raising=False)
+
+    messages = [
+        {
+            "role": "user",
+            "content": [{"image": {"format": "webp", "source": {"bytes": b"webpimage"}}}],
+        },
+    ]
+
+    tru_request = model.format_request(messages)
+
+    assert tru_request["messages"][0]["content"][0]["source"]["media_type"] == "image/webp"
+
+
 def test_format_request_with_reasoning(model, model_id, max_tokens):
     messages = [
         {