diff --git a/src/strands/session/s3_session_manager.py b/src/strands/session/s3_session_manager.py
index 8d557e81c..e956f7be3 100644
--- a/src/strands/session/s3_session_manager.py
+++ b/src/strands/session/s3_session_manager.py
@@ -1,5 +1,6 @@
 """S3-based session manager for cloud storage."""
 
+import base64
 import json
 import logging
 from concurrent.futures import ThreadPoolExecutor, as_completed
@@ -25,6 +26,32 @@
 MESSAGE_PREFIX = "message_"
 MULTI_AGENT_PREFIX = "multi_agent_"
 
+_BYTES_TYPE_TAG = "__bytes_encoded__"
+
+
+class _BytesEncoder(json.JSONEncoder):
+    """JSON encoder that handles bytes objects by base64-encoding them.
+
+    Uses the same tagging convention as :func:`~strands.types.session.encode_bytes_values`
+    (``{"__bytes_encoded__": True, "data": "<base64>"}``), so the data can be
+    decoded by :func:`~strands.types.session.decode_bytes_values` transparently.
+    """
+
+    def default(self, o: Any) -> Any:
+        if isinstance(o, (bytes, bytearray)):
+            return {_BYTES_TYPE_TAG: True, "data": base64.b64encode(o).decode("ascii")}
+        return super().default(o)
+
+
+def _bytes_decoder_hook(obj: dict[str, Any]) -> Any:
+    """``object_hook`` for :func:`json.loads` that restores base64-encoded bytes.
+
+    Matches the tagging convention used by ``encode_bytes_values``.
+    """
+    if obj.get(_BYTES_TYPE_TAG) is True and "data" in obj:
+        return base64.b64decode(obj["data"])
+    return obj
+
 
 class S3SessionManager(RepositorySessionManager, SessionRepository):
     """S3-based session manager for cloud storage.
@@ -136,7 +163,7 @@ def _read_s3_object(self, key: str) -> dict[str, Any] | None:
         try:
             response = self.client.get_object(Bucket=self.bucket, Key=key)
             content = response["Body"].read().decode("utf-8")
-            return cast(dict[str, Any], json.loads(content))
+            return cast(dict[str, Any], json.loads(content, object_hook=_bytes_decoder_hook))
         except ClientError as e:
             if e.response["Error"]["Code"] == "NoSuchKey":
                 return None
@@ -148,7 +175,7 @@ def _read_s3_object(self, key: str) -> dict[str, Any] | None:
     def _write_s3_object(self, key: str, data: dict[str, Any]) -> None:
         """Write JSON object to S3."""
         try:
-            content = json.dumps(data, indent=2, ensure_ascii=False)
+            content = json.dumps(data, cls=_BytesEncoder, indent=2, ensure_ascii=False)
             self.client.put_object(
                 Bucket=self.bucket, Key=key, Body=content.encode("utf-8"), ContentType="application/json"
             )
diff --git a/tests/strands/session/test_s3_session_manager.py b/tests/strands/session/test_s3_session_manager.py
index c1c89da5b..22ceb2e75 100644
--- a/tests/strands/session/test_s3_session_manager.py
+++ b/tests/strands/session/test_s3_session_manager.py
@@ -481,3 +481,45 @@ def test_update_nonexistent_multi_agent(s3_manager, sample_session):
     nonexistent_mock.id = "nonexistent"
     with pytest.raises(SessionException):
         s3_manager.update_multi_agent(sample_session.session_id, nonexistent_mock)
+
+
+def test_write_and_read_message_with_binary_document_content(s3_manager, sample_session, sample_agent, sample_message):
+    """Test that messages containing bytes (e.g., inline PDF documents) can be serialized and deserialized.
+
+    Reproduces: https://github.com/strands-agents/sdk-python/issues/1864
+    """
+    # Create session and agent
+    s3_manager.create_session(sample_session)
+    s3_manager.create_agent(sample_session.session_id, sample_agent)
+
+    # Create a message containing binary document content (simulating a multimodal prompt)
+    pdf_bytes = b"%PDF-1.4 fake content for test"
+    message_with_bytes = SessionMessage.from_message(
+        message={
+            "role": "user",
+            "content": [
+                {"text": "Analyze this PDF"},
+                {
+                    "document": {
+                        "format": "pdf",
+                        "name": "document.pdf",
+                        "source": {
+                            "bytes": pdf_bytes,
+                        },
+                    }
+                },
+            ],
+        },
+        index=0,
+    )
+
+    # This should NOT raise TypeError: Object of type bytes is not JSON serializable
+    s3_manager.create_message(sample_session.session_id, sample_agent.agent_id, message_with_bytes)
+
+    # Read back and verify the bytes are correctly restored
+    result = s3_manager.read_message(sample_session.session_id, sample_agent.agent_id, 0)
+    assert result is not None
+    content = result.message["content"]
+    assert len(content) == 2
+    assert content[0]["text"] == "Analyze this PDF"
+    assert content[1]["document"]["source"]["bytes"] == pdf_bytes