diff --git a/changelog/630.fixed.md b/changelog/630.fixed.md
new file mode 100644
index 00000000..34a54db3
--- /dev/null
+++ b/changelog/630.fixed.md
@@ -0,0 +1 @@
+Fixed SDK including explicit `null` values for uninitialized optional relationships when creating nodes with object templates, which prevented the backend from applying template defaults.
diff --git a/infrahub_sdk/node/node.py b/infrahub_sdk/node/node.py
index 72624467..16e55ebf 100644
--- a/infrahub_sdk/node/node.py
+++ b/infrahub_sdk/node/node.py
@@ -192,7 +192,7 @@ def is_resource_pool(self) -> bool:
     def get_raw_graphql_data(self) -> dict | None:
         return self._data
 
-    def _generate_input_data(  # noqa: C901
+    def _generate_input_data(  # noqa: C901, PLR0915
         self,
         exclude_unmodified: bool = False,
         exclude_hfid: bool = False,
@@ -235,7 +235,10 @@ def _generate_input_data(  # noqa: C901
             rel: RelatedNodeBase | RelationshipManagerBase = getattr(self, item_name)
 
             if rel_schema.cardinality == RelationshipCardinality.ONE and rel_schema.optional and not rel.initialized:
-                data[item_name] = None
+                # Only include None for existing nodes to allow clearing relationships
+                # For new nodes, omit the field to allow object template defaults to be applied
+                if self._existing:
+                    data[item_name] = None
                 continue
 
             if rel is None or not rel.initialized:
diff --git a/tests/unit/sdk/test_node.py b/tests/unit/sdk/test_node.py
index e4192871..810bd6a6 100644
--- a/tests/unit/sdk/test_node.py
+++ b/tests/unit/sdk/test_node.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import inspect
 import ipaddress
 from typing import TYPE_CHECKING
@@ -16,10 +18,14 @@
 )
 from infrahub_sdk.node.constants import SAFE_VALUE
 from infrahub_sdk.node.related_node import RelatedNode, RelatedNodeSync
-from infrahub_sdk.schema import GenericSchema, NodeSchemaAPI
 
 if TYPE_CHECKING:
+    from pytest_httpx import HTTPXMock
+
     from infrahub_sdk.client import InfrahubClient, InfrahubClientSync
+    from infrahub_sdk.schema import GenericSchema, NodeSchemaAPI
+
+    from .conftest import BothClients
 
 # type: ignore[attr-defined]
 
@@ -1365,7 +1371,6 @@ async def test_create_input_data(client, location_schema: NodeSchemaAPI, client_
             "name": {"value": "JFK1"},
             "description": {"value": "JFK Airport"},
             "type": {"value": "SITE"},
-            "primary_tag": None,
         }
     }
 
@@ -1393,6 +1398,38 @@ async def test_create_input_data_with_dropdown(client, location_schema_with_drop
             "description": {"value": "JFK Airport"},
             "type": {"value": "SITE"},
             "status": {"value": None},
+        }
+    }
+
+
+@pytest.mark.parametrize("client_type", client_types)
+async def test_update_input_data_existing_node_with_optional_relationship(
+    clients: BothClients, location_schema: NodeSchemaAPI, client_type: str
+) -> None:
+    """Validate that existing nodes include None for uninitialized optional relationships.
+
+    This ensures that we can explicitly clear optional relationships when updating existing nodes.
+    """
+    # Simulate an existing node by including an id
+    data = {
+        "id": "existing-node-id",
+        "name": {"value": "JFK1"},
+        "description": {"value": "JFK Airport"},
+        "type": {"value": "SITE"},
+    }
+
+    if client_type == "standard":
+        node = InfrahubNode(client=clients.standard, schema=location_schema, data=data)
+    else:
+        node = InfrahubNodeSync(client=clients.sync, schema=location_schema, data=data)
+
+    # For existing nodes, optional uninitialized relationships should include None
+    assert node._generate_input_data()["data"] == {
+        "data": {
+            "id": "existing-node-id",
+            "name": {"value": "JFK1"},
+            "description": {"value": "JFK Airport"},
+            "type": {"value": "SITE"},
             "primary_tag": None,
         }
     }
@@ -1641,7 +1678,7 @@ async def test_create_input_data_with_IPHost_attribute(client, ipaddress_schema,
         ip_address = InfrahubNodeSync(client=client, schema=ipaddress_schema, data=data)
 
     assert ip_address._generate_input_data()["data"] == {
-        "data": {"address": {"value": "1.1.1.1/24", "is_protected": True}, "interface": None}
+        "data": {"address": {"value": "1.1.1.1/24", "is_protected": True}}
     }
 
 
@@ -1655,7 +1692,7 @@ async def test_create_input_data_with_IPNetwork_attribute(client, ipnetwork_sche
         ip_network = InfrahubNodeSync(client=client, schema=ipnetwork_schema, data=data)
 
     assert ip_network._generate_input_data()["data"] == {
-        "data": {"network": {"value": "1.1.1.0/24", "is_protected": True}, "site": None}
+        "data": {"network": {"value": "1.1.1.0/24", "is_protected": True}}
     }