diff --git a/haystack/components/retrievers/in_memory/bm25_retriever.py b/haystack/components/retrievers/in_memory/bm25_retriever.py
index 05618f7d810..450f70cc561 100644
--- a/haystack/components/retrievers/in_memory/bm25_retriever.py
+++ b/haystack/components/retrievers/in_memory/bm25_retriever.py
@@ -6,7 +6,7 @@
 
 from haystack import Document, component, default_from_dict, default_to_dict
 from haystack.document_stores.in_memory import InMemoryDocumentStore
-from haystack.document_stores.types import FilterPolicy
+from haystack.document_stores.types import FilterPolicy, apply_filter_policy
 
 
 @component
@@ -143,10 +143,7 @@ def run(
         :raises ValueError:
             If the specified DocumentStore is not found or is not a InMemoryDocumentStore instance.
         """
-        if self.filter_policy == FilterPolicy.MERGE and filters:
-            filters = {**(self.filters or {}), **filters}
-        else:
-            filters = filters or self.filters
+        filters = apply_filter_policy(self.filter_policy, self.filters, filters)
         if top_k is None:
             top_k = self.top_k
         if scale_score is None:
@@ -181,10 +178,7 @@ async def run_async(
         :raises ValueError:
             If the specified DocumentStore is not found or is not a InMemoryDocumentStore instance.
         """
-        if self.filter_policy == FilterPolicy.MERGE and filters:
-            filters = {**(self.filters or {}), **filters}
-        else:
-            filters = filters or self.filters
+        filters = apply_filter_policy(self.filter_policy, self.filters, filters)
         if top_k is None:
             top_k = self.top_k
         if scale_score is None:
diff --git a/haystack/components/retrievers/in_memory/embedding_retriever.py b/haystack/components/retrievers/in_memory/embedding_retriever.py
index f185d33dc2f..5c2bf0dcf8f 100644
--- a/haystack/components/retrievers/in_memory/embedding_retriever.py
+++ b/haystack/components/retrievers/in_memory/embedding_retriever.py
@@ -6,7 +6,7 @@
 
 from haystack import Document, component, default_from_dict, default_to_dict
 from haystack.document_stores.in_memory import InMemoryDocumentStore
-from haystack.document_stores.types import FilterPolicy
+from haystack.document_stores.types import FilterPolicy, apply_filter_policy
 
 
 @component
@@ -163,10 +163,7 @@ def run(
         :raises ValueError:
             If the specified DocumentStore is not found or is not an InMemoryDocumentStore instance.
         """
-        if self.filter_policy == FilterPolicy.MERGE and filters:
-            filters = {**(self.filters or {}), **filters}
-        else:
-            filters = filters or self.filters
+        filters = apply_filter_policy(self.filter_policy, self.filters, filters)
         if top_k is None:
             top_k = self.top_k
         if scale_score is None:
@@ -214,10 +211,7 @@ async def run_async(
         :raises ValueError:
             If the specified DocumentStore is not found or is not an InMemoryDocumentStore instance.
         """
-        if self.filter_policy == FilterPolicy.MERGE and filters:
-            filters = {**(self.filters or {}), **filters}
-        else:
-            filters = filters or self.filters
+        filters = apply_filter_policy(self.filter_policy, self.filters, filters)
         if top_k is None:
             top_k = self.top_k
         if scale_score is None:
diff --git a/releasenotes/notes/filter-policy-merge-in-memory-retrievers-f49dfcb2a1332e8b.yaml b/releasenotes/notes/filter-policy-merge-in-memory-retrievers-f49dfcb2a1332e8b.yaml
new file mode 100644
index 00000000000..c4eb313961c
--- /dev/null
+++ b/releasenotes/notes/filter-policy-merge-in-memory-retrievers-f49dfcb2a1332e8b.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix ``FilterPolicy.MERGE`` in ``InMemoryBM25Retriever`` and
+    ``InMemoryEmbeddingRetriever`` so initialization filters are combined with
+    runtime comparison filters instead of being silently overwritten.
\ No newline at end of file
diff --git a/test/components/retrievers/test_in_memory_bm25_retriever.py b/test/components/retrievers/test_in_memory_bm25_retriever.py
index 42cbcb56ce8..3452667d95d 100644
--- a/test/components/retrievers/test_in_memory_bm25_retriever.py
+++ b/test/components/retrievers/test_in_memory_bm25_retriever.py
@@ -139,6 +139,47 @@ def test_retriever_valid_run(self, in_memory_doc_store, mock_docs):
         assert len(result["documents"]) == 5
         assert result["documents"][0].content == "PHP is a popular programming language"
 
+    def test_run_with_filter_policy_merge_combines_init_and_runtime_filters(self, in_memory_doc_store):
+        in_memory_doc_store.write_documents(
+            [
+                Document(content="python article current", meta={"type": "article", "year": 2020}),
+                Document(content="python blog current", meta={"type": "blog", "year": 2021}),
+                Document(content="python article archived", meta={"type": "article", "year": 2019}),
+            ]
+        )
+
+        retriever = InMemoryBM25Retriever(
+            in_memory_doc_store,
+            filters={"field": "meta.type", "operator": "==", "value": "article"},
+            filter_policy=FilterPolicy.MERGE,
+        )
+
+        result = retriever.run(query="python", filters={"field": "meta.year", "operator": ">=", "value": 2020})
+
+        assert [doc.content for doc in result["documents"]] == ["python article current"]
+
+    @pytest.mark.asyncio
+    async def test_run_async_with_filter_policy_merge_combines_init_and_runtime_filters(self, in_memory_doc_store):
+        in_memory_doc_store.write_documents(
+            [
+                Document(content="python article current", meta={"type": "article", "year": 2020}),
+                Document(content="python blog current", meta={"type": "blog", "year": 2021}),
+                Document(content="python article archived", meta={"type": "article", "year": 2019}),
+            ]
+        )
+
+        retriever = InMemoryBM25Retriever(
+            in_memory_doc_store,
+            filters={"field": "meta.type", "operator": "==", "value": "article"},
+            filter_policy=FilterPolicy.MERGE,
+        )
+
+        result = await retriever.run_async(
+            query="python", filters={"field": "meta.year", "operator": ">=", "value": 2020}
+        )
+
+        assert [doc.content for doc in result["documents"]] == ["python article current"]
+
     def test_invalid_run_wrong_store_type(self):
         SomeOtherDocumentStore = document_store_class("SomeOtherDocumentStore")
         with pytest.raises(TypeError, match="document_store must be an instance of InMemoryDocumentStore"):
diff --git a/test/components/retrievers/test_in_memory_embedding_retriever.py b/test/components/retrievers/test_in_memory_embedding_retriever.py
index 024495c312b..4cf6d9dceef 100644
--- a/test/components/retrievers/test_in_memory_embedding_retriever.py
+++ b/test/components/retrievers/test_in_memory_embedding_retriever.py
@@ -141,6 +141,67 @@ def test_valid_run(self):
         assert len(result["documents"]) == top_k
         assert result["documents"][0].embedding == [1.0, 1.0, 1.0, 1.0]
 
+    def test_run_with_filter_policy_merge_combines_init_and_runtime_filters(self):
+        ds = InMemoryDocumentStore(embedding_similarity_function="cosine")
+        ds.write_documents(
+            [
+                Document(
+                    content="python article current",
+                    embedding=[1.0, 0.0, 0.0, 0.0],
+                    meta={"type": "article", "year": 2020},
+                ),
+                Document(
+                    content="python blog current", embedding=[1.0, 0.0, 0.0, 0.0], meta={"type": "blog", "year": 2021}
+                ),
+                Document(
+                    content="python article archived",
+                    embedding=[1.0, 0.0, 0.0, 0.0],
+                    meta={"type": "article", "year": 2019},
+                ),
+            ]
+        )
+
+        retriever = InMemoryEmbeddingRetriever(
+            ds, filters={"field": "meta.type", "operator": "==", "value": "article"}, filter_policy=FilterPolicy.MERGE
+        )
+
+        result = retriever.run(
+            query_embedding=[1.0, 0.0, 0.0, 0.0], filters={"field": "meta.year", "operator": ">=", "value": 2020}
+        )
+
+        assert [doc.content for doc in result["documents"]] == ["python article current"]
+
+    @pytest.mark.asyncio
+    async def test_run_async_with_filter_policy_merge_combines_init_and_runtime_filters(self):
+        ds = InMemoryDocumentStore(embedding_similarity_function="cosine")
+        ds.write_documents(
+            [
+                Document(
+                    content="python article current",
+                    embedding=[1.0, 0.0, 0.0, 0.0],
+                    meta={"type": "article", "year": 2020},
+                ),
+                Document(
+                    content="python blog current", embedding=[1.0, 0.0, 0.0, 0.0], meta={"type": "blog", "year": 2021}
+                ),
+                Document(
+                    content="python article archived",
+                    embedding=[1.0, 0.0, 0.0, 0.0],
+                    meta={"type": "article", "year": 2019},
+                ),
+            ]
+        )
+
+        retriever = InMemoryEmbeddingRetriever(
+            ds, filters={"field": "meta.type", "operator": "==", "value": "article"}, filter_policy=FilterPolicy.MERGE
+        )
+
+        result = await retriever.run_async(
+            query_embedding=[1.0, 0.0, 0.0, 0.0], filters={"field": "meta.year", "operator": ">=", "value": 2020}
+        )
+
+        assert [doc.content for doc in result["documents"]] == ["python article current"]
+
     def test_invalid_run_wrong_store_type(self):
         SomeOtherDocumentStore = document_store_class("SomeOtherDocumentStore")
         with pytest.raises(TypeError, match="document_store must be an instance of InMemoryDocumentStore"):