diff --git a/.semversioner/next-release/patch-20260329050537886994.json b/.semversioner/next-release/patch-20260329050537886994.json
new file mode 100644
index 0000000000..2a28daea11
--- /dev/null
+++ b/.semversioner/next-release/patch-20260329050537886994.json
@@ -0,0 +1,4 @@
+{
+  "type": "patch",
+  "description": "Add XML declaration header to GraphML snapshot for Gephi compatibility"
+}
diff --git a/packages/graphrag/graphrag/index/operations/snapshot_graphml.py b/packages/graphrag/graphrag/index/operations/snapshot_graphml.py
index 16b2d1d574..f7ea218f2f 100644
--- a/packages/graphrag/graphrag/index/operations/snapshot_graphml.py
+++ b/packages/graphrag/graphrag/index/operations/snapshot_graphml.py
@@ -7,6 +7,8 @@
 import pandas as pd
 from graphrag_storage import Storage
 
+_XML_HEADER = '<?xml version="1.0" encoding="utf-8"?>'
+
 
 async def snapshot_graphml(
     edges: pd.DataFrame,
@@ -15,5 +17,5 @@ async def snapshot_graphml(
 ) -> None:
     """Take a entire snapshot of a graph to standard graphml format."""
     graph = nx.from_pandas_edgelist(edges, edge_attr=["weight"])
-    graphml = "\n".join(nx.generate_graphml(graph))
+    graphml = _XML_HEADER + "\n" + "\n".join(nx.generate_graphml(graph))
     await storage.set(name + ".graphml", graphml)
diff --git a/tests/unit/indexing/test_snapshot_graphml.py b/tests/unit/indexing/test_snapshot_graphml.py
new file mode 100644
index 0000000000..3b8e7e0fed
--- /dev/null
+++ b/tests/unit/indexing/test_snapshot_graphml.py
@@ -0,0 +1,96 @@
+# Copyright (C) 2026 Microsoft
+# Licensed under the MIT License
+
+"""Tests for snapshot_graphml operation."""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import TYPE_CHECKING, Any
+
+import pandas as pd
+from graphrag.index.operations.snapshot_graphml import (
+    _XML_HEADER,
+    snapshot_graphml,
+)
+from graphrag_storage import Storage
+
+if TYPE_CHECKING:
+    import re
+    from collections.abc import Iterator
+
+
+class FakeStorage(Storage):
+    """In-memory storage for testing."""
+
+    def __init__(self, **kwargs: Any) -> None:
+        self.data: dict[str, str] = {}
+
+    async def set(self, key: str, value: Any, encoding: str | None = None) -> None:
+        self.data[key] = value
+
+    def find(self, file_pattern: re.Pattern[str]) -> Iterator[str]:
+        yield from []
+
+    async def get(
+        self, key: str, as_bytes: bool | None = None, encoding: str | None = None
+    ) -> Any:
+        return self.data.get(key)
+
+    async def has(self, key: str) -> bool:
+        return key in self.data
+
+    async def delete(self, key: str) -> None:
+        self.data.pop(key, None)
+
+    async def clear(self) -> None:
+        self.data.clear()
+
+    def child(self, name: str | None) -> FakeStorage:
+        return FakeStorage()
+
+    def keys(self) -> list[str]:
+        return list(self.data.keys())
+
+    async def get_creation_date(self, key: str) -> str:
+        return datetime.now(timezone.utc).strftime("%Y-%m-%d %H:%M:%S %z")
+
+
+def _make_edges(
+    rows: list[tuple[str, str, float]],
+) -> pd.DataFrame:
+    """Build a minimal relationships DataFrame."""
+    return pd.DataFrame(rows, columns=["source", "target", "weight"])
+
+
+class TestSnapshotGraphml:
+    """Tests for the snapshot_graphml function."""
+
+    async def test_output_has_xml_header(self) -> None:
+        """GraphML output must start with the XML declaration."""
+        edges = _make_edges([("A", "B", 1.0)])
+        storage = FakeStorage()
+        await snapshot_graphml(edges, "graph", storage)
+
+        content = storage.data["graph.graphml"]
+        assert content.startswith(_XML_HEADER)
+
+    async def test_output_is_valid_graphml(self) -> None:
+        """Output should contain <graphml> root element."""
+        edges = _make_edges([("A", "B", 1.0)])
+        storage = FakeStorage()
+        await snapshot_graphml(edges, "graph", storage)
+
+        content = storage.data["graph.graphml"]
+        assert "<graphml" in content
+        assert "</graphml>" in content
+
+    async def test_empty_graph(self) -> None:
+        """Empty edge list should produce valid GraphML with XML header."""
+        edges = _make_edges([])
+        storage = FakeStorage()
+        await snapshot_graphml(edges, "graph", storage)
+
+        content = storage.data["graph.graphml"]
+        assert content.startswith(_XML_HEADER)
+        assert "<graphml" in content