diff --git a/core/sync/s3_store.py b/core/sync/s3_store.py
new file mode 100644
index 00000000..6785d9ef
--- /dev/null
+++ b/core/sync/s3_store.py
@@ -0,0 +1,103 @@
+"""Optional S3-compatible object-store adapter.
+
+This module is intentionally outside CORE hot paths.  It imports boto3 lazily
+inside the adapter constructor so importing core.sync remains safe on systems
+without S3 dependencies installed.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from core.sync.object_store import ObjectMetadata, ObjectNotFoundError, ObjectStoreError
+
+
+class S3ObjectStore:
+    """ObjectStore implementation backed by an S3-compatible bucket."""
+
+    def __init__(
+        self,
+        *,
+        bucket: str,
+        client: Any | None = None,
+        endpoint_url: str | None = None,
+        region_name: str | None = None,
+    ) -> None:
+        if not bucket:
+            raise ValueError("bucket is required")
+        self.bucket = bucket
+        if client is not None:
+            self._client = client
+            return
+        try:
+            import boto3  # type: ignore[import-not-found]
+        except ImportError as exc:
+            raise ObjectStoreError("boto3 is not installed; install optional S3 dependencies") from exc
+        kwargs: dict[str, str] = {}
+        if endpoint_url is not None:
+            kwargs["endpoint_url"] = endpoint_url
+        if region_name is not None:
+            kwargs["region_name"] = region_name
+        self._client = boto3.client("s3", **kwargs)
+
+    def put_bytes(self, key: str, data: bytes, *, content_type: str) -> ObjectMetadata:
+        if not key:
+            raise ValueError("key is required")
+        try:
+            response = self._client.put_object(
+                Bucket=self.bucket,
+                Key=key,
+                Body=data,
+                ContentType=content_type,
+            )
+        except Exception as exc:  # noqa: BLE001 - provider SDK exceptions vary
+            raise ObjectStoreError(f"put_object failed for key {key!r}: {exc}") from exc
+        return ObjectMetadata(
+            key=key,
+            size_bytes=len(data),
+            content_type=content_type,
+            etag=_clean_etag(response.get("ETag")) if isinstance(response, dict) else None,
+        )
+
+    def get_bytes(self, key: str) -> bytes:
+        if not key:
+            raise ValueError("key is required")
+        try:
+            response = self._client.get_object(Bucket=self.bucket, Key=key)
+            body = response["Body"].read()
+        except Exception as exc:  # noqa: BLE001 - provider SDK exceptions vary
+            if _looks_not_found(exc):
+                raise ObjectNotFoundError(f"object not found: {key}") from exc
+            raise ObjectStoreError(f"get_object failed for key {key!r}: {exc}") from exc
+        if not isinstance(body, bytes):
+            raise ObjectStoreError(f"get_object returned non-bytes body for key {key!r}")
+        return body
+
+    def exists(self, key: str) -> bool:
+        if not key:
+            raise ValueError("key is required")
+        try:
+            self._client.head_object(Bucket=self.bucket, Key=key)
+            return True
+        except Exception as exc:  # noqa: BLE001 - provider SDK exceptions vary
+            if _looks_not_found(exc):
+                return False
+            raise ObjectStoreError(f"head_object failed for key {key!r}: {exc}") from exc
+
+
+def _clean_etag(value: object) -> str | None:
+    if not isinstance(value, str):
+        return None
+    return value.strip('"')
+
+
+def _looks_not_found(exc: Exception) -> bool:
+    response = getattr(exc, "response", None)
+    if isinstance(response, dict):
+        error = response.get("Error")
+        if isinstance(error, dict):
+            code = str(error.get("Code", ""))
+            return code in {"NoSuchKey", "404", "NotFound"}
+    name = exc.__class__.__name__.lower()
+    text = str(exc).lower()
+    return "nosuchkey" in name or "not found" in text or "404" in text