diff --git a/haystack/document_stores/elasticsearch.py b/haystack/document_stores/elasticsearch.py
index 673ef6ce6..b24ea44b6 100644
--- a/haystack/document_stores/elasticsearch.py
+++ b/haystack/document_stores/elasticsearch.py
@@ -231,6 +231,10 @@ class ElasticsearchDocumentStore(SearchEngineDocumentStore):
         elif aws4auth:
             # aws elasticsearch with IAM
             # see https://elasticsearch-py.readthedocs.io/en/v7.12.0/index.html?highlight=http_auth#running-on-aws-with-iam
+            if username:
+                logger.warning(
+                    "aws4auth and a username are passed to the ElasticsearchDocumentStore. The username will be ignored and aws4auth will be used for authentication."
+                )
             client = Elasticsearch(
                 hosts=hosts,
                 http_auth=aws4auth,
diff --git a/haystack/document_stores/opensearch.py b/haystack/document_stores/opensearch.py
index f1099089c..6adf391cc 100644
--- a/haystack/document_stores/opensearch.py
+++ b/haystack/document_stores/opensearch.py
@@ -238,7 +238,22 @@ class OpenSearchDocumentStore(SearchEngineDocumentStore):
         if use_system_proxy:
             connection_class = RequestsHttpConnection  # type: ignore [assignment]
 
-        if username:
+        if aws4auth:
+            # Sign requests to Opensearch with IAM credentials
+            # see https://docs.aws.amazon.com/opensearch-service/latest/developerguide/request-signing.html#request-signing-python
+            if username:
+                logger.warning(
+                    "aws4auth and a username or the default username 'admin' are passed to the OpenSearchDocumentStore. The username will be ignored and aws4auth will be used for authentication."
+                )
+            client = OpenSearch(
+                hosts=hosts,
+                http_auth=aws4auth,
+                connection_class=RequestsHttpConnection,
+                use_ssl=True,
+                verify_certs=True,
+                timeout=timeout,
+            )
+        elif username:
             # standard http_auth
             client = OpenSearch(
                 hosts=hosts,
@@ -249,17 +264,6 @@ class OpenSearchDocumentStore(SearchEngineDocumentStore):
                 timeout=timeout,
                 connection_class=connection_class,
             )
-        elif aws4auth:
-            # Sign requests to Opensearch with IAM credentials
-            # see https://docs.aws.amazon.com/opensearch-service/latest/developerguide/request-signing.html#request-signing-python
-            client = OpenSearch(
-                hosts=hosts,
-                http_auth=aws4auth,
-                connection_class=RequestsHttpConnection,
-                use_ssl=True,
-                verify_certs=True,
-                timeout=timeout,
-            )
         else:
             # no authentication needed
             client = OpenSearch(
diff --git a/test/document_stores/test_elasticsearch.py b/test/document_stores/test_elasticsearch.py
index cca979eff..5b9ee1577 100644
--- a/test/document_stores/test_elasticsearch.py
+++ b/test/document_stores/test_elasticsearch.py
@@ -1,11 +1,12 @@
+import logging
 import os
 from unittest.mock import MagicMock
-import pytest
 
 import numpy as np
+import pytest
 
+from haystack.document_stores.elasticsearch import ElasticsearchDocumentStore, Elasticsearch
 from haystack.schema import Document
-from haystack.document_stores.elasticsearch import ElasticsearchDocumentStore
 
 from .test_base import DocumentStoreBaseTestAbstract
 from .test_search_engine import SearchEngineDocumentStoreTestAbstract
@@ -32,6 +33,18 @@ class TestElasticsearchDocumentStore(DocumentStoreBaseTestAbstract, SearchEngine
         ds.delete_index(self.index_name)
         ds.delete_index(labels_index_name)
 
+    @pytest.fixture
+    def mocked_elastic_search_init(self, monkeypatch):
+        mocked_init = MagicMock(return_value=None)
+        monkeypatch.setattr(Elasticsearch, "__init__", mocked_init)
+        return mocked_init
+
+    @pytest.fixture
+    def mocked_elastic_search_ping(self, monkeypatch):
+        mocked_ping = MagicMock(return_value=True)
+        monkeypatch.setattr(Elasticsearch, "ping", mocked_ping)
+        return mocked_ping
+
     @pytest.fixture
     def mocked_document_store(self):
         """
@@ -239,3 +252,38 @@ class TestElasticsearchDocumentStore(DocumentStoreBaseTestAbstract, SearchEngine
         assert ds.get_document_count(only_documents_without_embedding=True) == 3
         assert ds.get_document_count(only_documents_without_embedding=True, filters={"month": ["01"]}) == 0
         assert ds.get_document_count(only_documents_without_embedding=True, filters={"month": ["03"]}) == 3
+
+    @pytest.mark.unit
+    def test__init_elastic_client_aws4auth_and_username_raises_warning(
+        self, caplog, mocked_elastic_search_init, mocked_elastic_search_ping
+    ):
+        _init_client_remaining_kwargs = {
+            "host": "host",
+            "port": 443,
+            "password": "pass",
+            "api_key_id": None,
+            "api_key": None,
+            "scheme": "https",
+            "ca_certs": None,
+            "verify_certs": True,
+            "timeout": 10,
+            "use_system_proxy": False,
+        }
+
+        with caplog.at_level(logging.WARN, logger="haystack.document_stores.elasticsearch"):
+            ElasticsearchDocumentStore._init_elastic_client(
+                username="admin", aws4auth="foo", **_init_client_remaining_kwargs
+            )
+        assert len(caplog.records) == 1
+        for r in caplog.records:
+            assert r.levelname == "WARNING"
+
+        caplog.clear()
+        with caplog.at_level(logging.WARN, logger="haystack.document_stores.elasticsearch"):
+            ElasticsearchDocumentStore._init_elastic_client(
+                username=None, aws4auth="foo", **_init_client_remaining_kwargs
+            )
+            ElasticsearchDocumentStore._init_elastic_client(
+                username="", aws4auth="foo", **_init_client_remaining_kwargs
+            )
+        assert len(caplog.records) == 0
diff --git a/test/document_stores/test_opensearch.py b/test/document_stores/test_opensearch.py
index 53a1d97a5..f043536be 100644
--- a/test/document_stores/test_opensearch.py
+++ b/test/document_stores/test_opensearch.py
@@ -199,6 +199,32 @@ class TestOpenSearchDocumentStore(DocumentStoreBaseTestAbstract, SearchEngineDoc
         for r in caplog.records:
             assert r.levelname == "WARNING"
 
+    @pytest.mark.unit
+    def test__init_client_aws4auth_and_username_raises_warning(self, mocked_open_search_init, caplog):
+        _init_client_remaining_kwargs = {
+            "host": "host",
+            "port": 443,
+            "password": "pass",
+            "scheme": "https",
+            "ca_certs": None,
+            "verify_certs": True,
+            "timeout": 10,
+            "use_system_proxy": False,
+        }
+
+        with caplog.at_level(logging.WARN, logger="haystack.document_stores.opensearch"):
+            OpenSearchDocumentStore._init_client(username="admin", aws4auth="foo", **_init_client_remaining_kwargs)
+            OpenSearchDocumentStore._init_client(username="bar", aws4auth="foo", **_init_client_remaining_kwargs)
+        assert len(caplog.records) == 2
+        for r in caplog.records:
+            assert r.levelname == "WARNING"
+
+        caplog.clear()
+        with caplog.at_level(logging.WARN, logger="haystack.document_stores.opensearch"):
+            OpenSearchDocumentStore._init_client(username=None, aws4auth="foo", **_init_client_remaining_kwargs)
+            OpenSearchDocumentStore._init_client(username="foo", aws4auth=None, **_init_client_remaining_kwargs)
+        assert len(caplog.records) == 0
+
     @pytest.mark.unit
     def test___init___connection_test_fails(self, mocked_document_store):
         failing_client = MagicMock()