@@ -146,7 +146,7 @@ test = [
     "pytest-benchmark>=3.4.1,<4",
     "pytest-asyncio<=0.24.0",
     "py>=1.11.0",
-    "testcontainers==4.9.0",
+    "testcontainers>=4.15.0rc2",
     "minio==7.2.11",
     "python-keycloak==4.2.2",
     "cryptography>=43.0",
 
@@ -1,28 +1,33 @@
 from __future__ import annotations
 
+import time
 from datetime import datetime
 from logging import getLogger
-from typing import Any, Callable, Dict, List, Literal, Optional, Sequence, Tuple
+from typing import Any, Callable, Dict, List, Literal, Optional, Sequence, Tuple, Union
 
 try:
     from pymongo import AsyncMongoClient, MongoClient, UpdateOne
     from pymongo.asynchronous.collection import AsyncCollection
     from pymongo.collection import Collection
     from pymongo.driver_info import DriverInfo
+    from pymongo.operations import SearchIndexModel
 except ImportError as e:
     from feast.errors import FeastExtrasDependencyImportError
 
     raise FeastExtrasDependencyImportError("mongodb", str(e))
 
 import feast.version
+from feast.batch_feature_view import BatchFeatureView
 from feast.entity import Entity
 from feast.feature_view import FeatureView
-from feast.infra.key_encoding_utils import serialize_entity_key
+from feast.infra.key_encoding_utils import deserialize_entity_key, serialize_entity_key
 from feast.infra.online_stores.online_store import OnlineStore
+from feast.infra.online_stores.vector_store import VectorStoreConfig
 from feast.infra.supported_async_methods import SupportedAsyncMethods
 from feast.protos.feast.types.EntityKey_pb2 import EntityKey as EntityKeyProto
 from feast.protos.feast.types.Value_pb2 import Value as ValueProto
 from feast.repo_config import FeastConfigBaseModel, RepoConfig
+from feast.stream_feature_view import StreamFeatureView
 from feast.type_map import (
     feast_value_type_to_python_type,
     python_values_to_proto_values,
@@ -33,7 +38,7 @@
 DRIVER_METADATA = DriverInfo(name="Feast", version=feast.version.get_version())
 
 
-class MongoDBOnlineStoreConfig(FeastConfigBaseModel):
+class MongoDBOnlineStoreConfig(FeastConfigBaseModel, VectorStoreConfig):
     """MongoDB configuration.
 
     For a description of kwargs that may be passed to MongoClient,
@@ -48,6 +53,11 @@ class MongoDBOnlineStoreConfig(FeastConfigBaseModel):
     )
     collection_suffix: str = "latest"
     client_kwargs: Dict[str, Any] = {}
+    # vector_enabled and similarity are inherited from VectorStoreConfig
+    vector_index_wait_timeout: int = 60
+    """Seconds to wait for a newly created Atlas Search index to become READY."""
+    vector_index_wait_poll_interval: float = 1.0
+    """Seconds between polls when waiting for an Atlas Search index to become READY."""
 
 
 class MongoDBOnlineStore(OnlineStore):
@@ -213,11 +223,135 @@ def online_read(
 
         return self._convert_raw_docs_to_proto(ids, docs, table)
 
+    def retrieve_online_documents_v2(
+        self,
+        config: RepoConfig,
+        table: FeatureView,
+        requested_features: List[str],
+        embedding: Optional[List[float]],
+        top_k: int,
+        distance_metric: Optional[str] = None,
+        query_string: Optional[str] = None,
+        include_feature_view_version_metadata: bool = False,
+    ) -> List[
+        Tuple[
+            Optional[datetime],
+            Optional[EntityKeyProto],
+            Optional[Dict[str, ValueProto]],
+        ]
+    ]:
+        """Retrieve documents via MongoDB Atlas Vector Search ($vectorSearch).
+
+        Uses the ``$vectorSearch`` aggregation stage to find the *top_k*
+        documents closest to the provided *embedding* vector.  The method
+        expects that an Atlas vector search index has already been created for
+        the relevant field (see ``update()``).
+
+        Returns a list of 3-tuples ``(event_timestamp, entity_key_proto,
+        feature_dict)`` where *feature_dict* includes the requested feature
+        values plus a synthetic ``distance`` key with the vector search score.
+        """
+        if not config.online_store.vector_enabled:
+            raise ValueError(
+                "Vector search is not enabled in the online store config. "
+                "Set vector_enabled=True in MongoDBOnlineStoreConfig."
+            )
+        if embedding is None:
+            raise ValueError(
+                "An embedding vector must be provided for MongoDB vector search."
+            )
+
+        clxn = self._get_collection(config)
+
+        # Identify the vector field on this feature view
+        vector_fields = [f for f in table.features if f.vector_index]
+        if not vector_fields:
+            raise ValueError(
+                f"Feature view '{table.name}' has no fields with vector_index=True."
+            )
+        vector_field = vector_fields[0]
+        path = f"features.{table.name}.{vector_field.name}"
+        idx_name = self._vector_search_index_name(table.name, vector_field.name)
+
+        # BSON cannot encode numpy float types — ensure native Python floats.
+        query_vector = [float(v) for v in embedding]
+
+        num_candidates = max(top_k * 10, 100)
+        pipeline: List[Dict[str, Any]] = [
+            {
+                "$vectorSearch": {
+                    "index": idx_name,
+                    "path": path,
+                    "queryVector": query_vector,
+                    "numCandidates": num_candidates,
+                    "limit": top_k,
+                }
+            },
+            {
+                "$addFields": {
+                    "score": {"$meta": "vectorSearchScore"},
+                }
+            },
+        ]
+
+        results: List[
+            Tuple[
+                Optional[datetime],
+                Optional[EntityKeyProto],
+                Optional[Dict[str, ValueProto]],
+            ]
+        ] = []
+
+        for doc in clxn.aggregate(pipeline):
+            # Deserialize entity key
+            entity_key_bin = doc.get("_id")
+            entity_key_proto = (
+                deserialize_entity_key(
+                    entity_key_bin,
+                    entity_key_serialization_version=config.entity_key_serialization_version,
+                )
+                if entity_key_bin
+                else None
+            )
+
+            # Event timestamp
+            event_ts = doc.get("event_timestamps", {}).get(table.name)
+
+            # Build feature dict from raw doc values
+            fv_features = doc.get("features", {}).get(table.name, {})
+
+            # Convert raw values → ValueProto for each requested feature
+            feature_dict: Dict[str, ValueProto] = {}
+            feature_type_map = {f.name: f.dtype.to_value_type() for f in table.features}
+
+            for feat_name in requested_features:
+                raw_val = fv_features.get(feat_name)
+                if raw_val is not None:
+                    vtype = feature_type_map.get(feat_name)
+                    if vtype is not None:
+                        protos = python_values_to_proto_values(
+                            [raw_val], feature_type=vtype
+                        )
+                        feature_dict[feat_name] = protos[0]
+                    else:
+                        # Fall back: try to store as-is
+                        feature_dict[feat_name] = _python_value_to_proto(raw_val)
+
+            # Add distance (vector search score)
+            score = doc.get("score", 0.0)
+            feature_dict["distance"] = ValueProto(float_val=float(score))
+
+            results.append((event_ts, entity_key_proto, feature_dict))
+
+        return results
+
     def update(
         self,
         config: RepoConfig,
         tables_to_delete: Sequence[FeatureView],
-        tables_to_keep: Sequence[FeatureView],
+        tables_to_keep: Sequence[
+            Union[BatchFeatureView, StreamFeatureView, FeatureView]
+        ],
         entities_to_delete: Sequence[Entity],
         entities_to_keep: Sequence[Entity],
         partial: bool,
@@ -238,20 +372,32 @@ def update(
         }
         We remove any feature views named in tables_to_delete.
         The Entities are serialized in the _id. No schema needs be adjusted.
+
+        When ``vector_enabled`` is set in the online store config, Atlas Vector
+        Search indexes are automatically created for feature views containing
+        fields with ``vector_index=True`` and dropped for deleted feature views.
         """
         if not isinstance(config.online_store, MongoDBOnlineStoreConfig):
             raise RuntimeError(f"{config.online_store.type = }. It must be mongodb.")
 
+        online_config = config.online_store
         clxn = self._get_collection(repo_config=config)
 
+        # --- Remove deleted feature views (data + vector search indexes) ---
         if tables_to_delete:
             unset_fields = {}
             for fv in tables_to_delete:
                 unset_fields[f"features.{fv.name}"] = ""
                 unset_fields[f"event_timestamps.{fv.name}"] = ""
-
             clxn.update_many({}, {"$unset": unset_fields})
 
+            if online_config.vector_enabled:
+                self._drop_vector_indexes_for_tables(clxn, tables_to_delete)
+
+        # --- Create vector search indexes for kept feature views ---
+        if online_config.vector_enabled:
+            self._ensure_vector_indexes(clxn, tables_to_keep, online_config)
+
         # Note: entities_to_delete contains Entity definitions (metadata), not entity instances.
         # Like other online stores, we don't need to do anything with entities_to_delete here.
 
@@ -286,6 +432,136 @@ async def close(self) -> None:
     # Helpers
     # ------------------------------------------------------------------
 
+    # -- Vector Search helpers ------------------------------------------
+
+    @staticmethod
+    def _vector_search_index_name(fv_name: str, field_name: str) -> str:
+        """Canonical Atlas vector search index name for a (feature_view, field) pair."""
+        return f"{fv_name}__{field_name}__vs_index"
+
+    @staticmethod
+    def _vector_search_index_definition(
+        path: str,
+        num_dimensions: int,
+        similarity: str,
+    ) -> dict:
+        """Return a vector search index definition for ``SearchIndexModel``."""
+        return {
+            "fields": [
+                {
+                    "type": "vector",
+                    "path": path,
+                    "numDimensions": num_dimensions,
+                    "similarity": similarity,
+                }
+            ]
+        }
+
+    @staticmethod
+    def _wait_for_index_ready(
+        collection: Collection,
+        index_name: str,
+        timeout: int,
+        poll_interval: float = 1.0,
+    ) -> None:
+        """Poll until the named Atlas Search index reaches READY status.
+
+        Raises ``TimeoutError`` if the index does not become queryable
+        within *timeout* seconds.
+        """
+        deadline = time.monotonic() + timeout
+        while time.monotonic() < deadline:
+            for idx in collection.list_search_indexes(name=index_name):
+                if idx.get("status") == "READY" or idx.get("queryable") is True:
+                    return
+            time.sleep(poll_interval)
+        raise TimeoutError(
+            f"Atlas search index '{index_name}' did not reach READY "
+            f"within {timeout}s. Increase vector_index_wait_timeout in "
+            f"MongoDBOnlineStoreConfig if the index needs more time."
+        )
+
+    def _drop_vector_indexes_for_tables(
+        self,
+        collection: Collection,
+        tables: Sequence[FeatureView],
+    ) -> None:
+        """Drop all Atlas vector search indexes belonging to the given feature views."""
+        existing = {idx["name"] for idx in collection.list_search_indexes()}
+        for fv in tables:
+            for field in fv.features:
+                idx_name = self._vector_search_index_name(fv.name, field.name)
+                if idx_name in existing:
+                    logger.info("Dropping vector search index: %s", idx_name)
+                    collection.drop_search_index(idx_name)
+
+    def _ensure_vector_indexes(
+        self,
+        collection: Collection,
+        tables: Sequence[Union[BatchFeatureView, StreamFeatureView, FeatureView]],
+        online_config: MongoDBOnlineStoreConfig,
+    ) -> None:
+        """Create Atlas vector search indexes for vector-indexed fields if they don't exist.
+
+        Currently creates one index per (feature_view, vector_field) pair.
+        A future optimization could consolidate all vector fields into a
+        single composite index with multiple field definitions, reducing
+        cluster-wide index count and memory overhead.  See Atlas limits:
+        hard cap of 2,500 search indexes per cluster; smaller tiers (M10)
+        may degrade well before that.
+        """
+        db = collection.database
+        if collection.name not in db.list_collection_names():
+            db.create_collection(collection.name)
+
+        existing = {idx["name"] for idx in collection.list_search_indexes()}
+
+        for fv in tables:
+            vector_fields = [f for f in fv.features if f.vector_index]
+            for field in vector_fields:
+                idx_name = self._vector_search_index_name(fv.name, field.name)
+                if idx_name in existing:
+                    logger.debug("Vector search index '%s' already exists", idx_name)
+                    continue
+
+                path = f"features.{fv.name}.{field.name}"
+                num_dimensions = field.vector_length
+                if not num_dimensions:
+                    raise ValueError(
+                        f"Field '{field.name}' in feature view '{fv.name}' has "
+                        f"vector_index=True but vector_length is not set."
+                    )
+
+                similarity = (
+                    field.vector_search_metric or online_config.similarity or "cosine"
+                )
+
+                definition = self._vector_search_index_definition(
+                    path, num_dimensions, similarity
+                )
+                search_index_model = SearchIndexModel(
+                    definition=definition,
+                    name=idx_name,
+                    type="vectorSearch",
+                )
+                logger.info(
+                    "Creating Atlas vector search index '%s' on path '%s' "
+                    "(dims=%d, similarity=%s)",
+                    idx_name,
+                    path,
+                    num_dimensions,
+                    similarity,
+                )
+                collection.create_search_index(model=search_index_model)
+                self._wait_for_index_ready(
+                    collection,
+                    idx_name,
+                    online_config.vector_index_wait_timeout,
+                    online_config.vector_index_wait_poll_interval,
+                )
+
+    # -- Connection helpers ---------------------------------------------
+
     def _get_client(self, config: RepoConfig):
         """Returns a connection to the server."""
         online_store_config = config.online_store
@@ -494,5 +770,20 @@ async def online_write_batch_async(
             progress(len(data))
 
 
-# TODO
-#   - Vector Search (requires atlas image in testcontainers or similar)
+def _python_value_to_proto(value: Any) -> ValueProto:
+    """Best-effort conversion of a single Python value to ValueProto."""
+    if isinstance(value, float):
+        return ValueProto(float_val=value)
+    elif isinstance(value, bool):
+        return ValueProto(bool_val=value)
+    elif isinstance(value, int):
+        return ValueProto(int64_val=value)
+    elif isinstance(value, str):
+        return ValueProto(string_val=value)
+    elif isinstance(value, bytes):
+        return ValueProto(bytes_val=value)
+    elif isinstance(value, list) and all(isinstance(v, float) for v in value):
+        proto = ValueProto()
+        proto.float_list_val.val.extend(value)
+        return proto
+    return ValueProto()