diff --git a/haystack/document_stores/in_memory/document_store.py b/haystack/document_stores/in_memory/document_store.py
index 62c3fd496..f35fe56a9 100644
--- a/haystack/document_stores/in_memory/document_store.py
+++ b/haystack/document_stores/in_memory/document_store.py
@@ -4,6 +4,7 @@
 
 import math
 import re
+import uuid
 from collections import Counter
 from dataclasses import dataclass
 from typing import Any, Dict, Iterable, List, Literal, Optional, Tuple
@@ -42,6 +43,11 @@ class BM25DocumentStats:
     doc_len: int
 
 
+# Global storage for all InMemoryDocumentStore instances, indexed by the index name.
+_STORAGES: Dict[str, Dict[str, Document]] = {}
+_BM25_STATS_STORAGES: Dict[str, Dict[str, BM25DocumentStats]] = {}
+
+
 class InMemoryDocumentStore:
     """
     Stores data in-memory. It's ephemeral and cannot be saved to disk.
@@ -53,6 +59,7 @@ class InMemoryDocumentStore:
         bm25_algorithm: Literal["BM25Okapi", "BM25L", "BM25Plus"] = "BM25L",
         bm25_parameters: Optional[Dict] = None,
         embedding_similarity_function: Literal["dot_product", "cosine"] = "dot_product",
+        index: Optional[str] = None,
     ):
         """
         Initializes the DocumentStore.
@@ -66,11 +73,19 @@ class InMemoryDocumentStore:
         :param embedding_similarity_function: The similarity function used to compare Documents embeddings.
                                               One of "dot_product" (default) or "cosine".
                                               To choose the most appropriate function, look for information about your embedding model.
+        :param index: If specified uses a specific index to store the documents. If not specified, a random UUID is used.
+                      Using the same index allows you to store documents across multiple InMemoryDocumentStore instances.
         """
-        self.storage: Dict[str, Document] = {}
         self.bm25_tokenization_regex = bm25_tokenization_regex
         self.tokenizer = re.compile(bm25_tokenization_regex).findall
 
+        if index is None:
+            index = str(uuid.uuid4())
+
+        self.index = index
+        if self.index not in _STORAGES:
+            _STORAGES[self.index] = {}
+
         self.bm25_algorithm = bm25_algorithm
         self.bm25_algorithm_inst = self._dispatch_bm25()
         self.bm25_parameters = bm25_parameters or {}
@@ -81,7 +96,19 @@ class InMemoryDocumentStore:
         self._freq_vocab_for_idf: Counter = Counter()
 
         # Per-document statistics
-        self._bm25_attr: Dict[str, BM25DocumentStats] = {}
+        if self.index not in _BM25_STATS_STORAGES:
+            _BM25_STATS_STORAGES[self.index] = {}
+
+    @property
+    def storage(self) -> Dict[str, Document]:
+        """
+        Utility property that returns the storage used by this instance of InMemoryDocumentStore.
+        """
+        return _STORAGES.get(self.index, {})
+
+    @property
+    def _bm25_attr(self) -> Dict[str, BM25DocumentStats]:
+        return _BM25_STATS_STORAGES.get(self.index, {})
 
     def _dispatch_bm25(self):
         """
@@ -281,6 +308,7 @@ class InMemoryDocumentStore:
             bm25_algorithm=self.bm25_algorithm,
             bm25_parameters=self.bm25_parameters,
             embedding_similarity_function=self.embedding_similarity_function,
+            index=self.index,
         )
 
     @classmethod
diff --git a/releasenotes/notes/in-memory-docstore-memory-share-82b75d018b3545fc.yaml b/releasenotes/notes/in-memory-docstore-memory-share-82b75d018b3545fc.yaml
new file mode 100644
index 000000000..d60997a41
--- /dev/null
+++ b/releasenotes/notes/in-memory-docstore-memory-share-82b75d018b3545fc.yaml
@@ -0,0 +1,19 @@
+---
+features:
+  - |
+    Add memory sharing between different instances of InMemoryDocumentStore.
+    Setting the same `index` argument as another instance will make sure that the memory is shared.
+    e.g.
+    ```python
+    index = "my_personal_index"
+    document_store_1 = InMemoryDocumentStore(index=index)
+    document_store_2 = InMemoryDocumentStore(index=index)
+
+    assert document_store_1.count_documents() == 0
+    assert document_store_2.count_documents() == 0
+
+    document_store_1.write_documents([Document(content="Hello world")])
+
+    assert document_store_1.count_documents() == 1
+    assert document_store_2.count_documents() == 1
+    ```
diff --git a/test/components/retrievers/test_filter_retriever.py b/test/components/retrievers/test_filter_retriever.py
index 20d3392f5..bef09e7a7 100644
--- a/test/components/retrievers/test_filter_retriever.py
+++ b/test/components/retrievers/test_filter_retriever.py
@@ -63,10 +63,13 @@ class TestFilterRetriever:
         }
 
     def test_to_dict_with_custom_init_parameters(self):
-        ds = InMemoryDocumentStore()
+        ds = InMemoryDocumentStore(index="test_to_dict_with_custom_init_parameters")
         serialized_ds = ds.to_dict()
 
-        component = FilterRetriever(document_store=InMemoryDocumentStore(), filters={"lang": "en"})
+        component = FilterRetriever(
+            document_store=InMemoryDocumentStore(index="test_to_dict_with_custom_init_parameters"),
+            filters={"lang": "en"},
+        )
         data = component.to_dict()
         assert data == {
             "type": "haystack.components.retrievers.filter_retriever.FilterRetriever",
diff --git a/test/components/retrievers/test_in_memory_bm25_retriever.py b/test/components/retrievers/test_in_memory_bm25_retriever.py
index ddc6454b4..59a88a8e8 100644
--- a/test/components/retrievers/test_in_memory_bm25_retriever.py
+++ b/test/components/retrievers/test_in_memory_bm25_retriever.py
@@ -60,11 +60,14 @@ class TestMemoryBM25Retriever:
         }
 
     def test_to_dict_with_custom_init_parameters(self):
-        ds = InMemoryDocumentStore()
+        ds = InMemoryDocumentStore(index="test_to_dict_with_custom_init_parameters")
         serialized_ds = ds.to_dict()
 
         component = InMemoryBM25Retriever(
-            document_store=InMemoryDocumentStore(), filters={"name": "test.txt"}, top_k=5, scale_score=True
+            document_store=InMemoryDocumentStore(index="test_to_dict_with_custom_init_parameters"),
+            filters={"name": "test.txt"},
+            top_k=5,
+            scale_score=True,
         )
         data = component.to_dict()
         assert data == {
diff --git a/test/document_stores/test_in_memory.py b/test/document_stores/test_in_memory.py
index d3a78aeff..2a8679502 100644
--- a/test/document_stores/test_in_memory.py
+++ b/test/document_stores/test_in_memory.py
@@ -32,6 +32,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
                 "bm25_algorithm": "BM25L",
                 "bm25_parameters": {},
                 "embedding_similarity_function": "dot_product",
+                "index": store.index,
             },
         }
 
@@ -41,6 +42,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
             bm25_algorithm="BM25Plus",
             bm25_parameters={"key": "value"},
             embedding_similarity_function="cosine",
+            index="my_cool_index",
         )
         data = store.to_dict()
         assert data == {
@@ -50,6 +52,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
                 "bm25_algorithm": "BM25Plus",
                 "bm25_parameters": {"key": "value"},
                 "embedding_similarity_function": "cosine",
+                "index": "my_cool_index",
             },
         }
 
@@ -61,6 +64,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
                 "bm25_tokenization_regex": "custom_regex",
                 "bm25_algorithm": "BM25Plus",
                 "bm25_parameters": {"key": "value"},
+                "index": "my_cool_index",
             },
         }
         store = InMemoryDocumentStore.from_dict(data)
@@ -68,6 +72,7 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
         assert store.tokenizer
         assert store.bm25_algorithm == "BM25Plus"
         assert store.bm25_parameters == {"key": "value"}
+        assert store.index == "my_cool_index"
 
     def test_invalid_bm25_algorithm(self):
         with pytest.raises(ValueError, match="BM25 algorithm 'invalid' is not supported"):
@@ -414,3 +419,28 @@ class TestMemoryDocumentStore(DocumentStoreBaseTests):  # pylint: disable=R0904
             embedding=[0.1, 0.1, 0.1, 0.1], documents=docs, scale_score=False
         )
         assert scores == [0.1, 0.4]
+
+    def test_multiple_document_stores_using_same_index(self):
+        index = "test_multiple_document_stores_using_same_index"
+        document_store_1 = InMemoryDocumentStore(index=index)
+        document_store_2 = InMemoryDocumentStore(index=index)
+
+        assert document_store_1.count_documents() == document_store_2.count_documents() == 0
+
+        doc_1 = Document(content="Hello world")
+        document_store_1.write_documents([doc_1])
+        assert document_store_1.count_documents() == document_store_2.count_documents() == 1
+
+        assert document_store_1.filter_documents() == document_store_2.filter_documents() == [doc_1]
+
+        doc_2 = Document(content="Hello another world")
+        document_store_2.write_documents([doc_2])
+        assert document_store_1.count_documents() == document_store_2.count_documents() == 2
+
+        assert document_store_1.filter_documents() == document_store_2.filter_documents() == [doc_1, doc_2]
+
+        document_store_1.delete_documents([doc_2.id])
+        assert document_store_1.count_documents() == document_store_2.count_documents() == 1
+
+        document_store_2.delete_documents([doc_1.id])
+        assert document_store_1.count_documents() == document_store_2.count_documents() == 0