diff --git a/src/ifcopenshell-python/ifcopenshell/api/nest/assign_object.py b/src/ifcopenshell-python/ifcopenshell/api/nest/assign_object.py
index b49adc72a5..227163f467 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/nest/assign_object.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/nest/assign_object.py
@@ -104,20 +104,20 @@ def assign_object(
         return
 
     ifc2x3 = file.schema == "IFC2X3"
+    related_objects_set = set(related_objects)
 
-    related_objects = set(settings["related_objects"])
-    relating_object = settings["relating_object"]
     if ifc2x3:
         is_nested_by = next((i for i in relating_object.IsDecomposedBy if i.is_a("IfcRelNests")), None)
     else:
         is_nested_by = next((i for i in relating_object.IsNestedBy), None)
 
+    # NOTE: maintain .RelatedObjects order as it has meaning in IFC
     previous_nests_rels: set[ifcopenshell.entity_instance] = set()
     objects_without_nests: list[ifcopenshell.entity_instance] = []
     objects_with_nests: list[ifcopenshell.entity_instance] = []
 
     # check if there is anything to change
-    for object in related_objects:
+    for object in related_objects_set:
         if ifc2x3:
             object_rel = next((i for i in object.Decomposes if i.is_a("IfcRelNests")), None)
         else:
@@ -143,7 +143,7 @@ def assign_object(
 
     # unassign elements from previous nests
     for nests in previous_nests_rels:
-        cur_related_objects = set(nests.RelatedObjects) - related_objects
+        cur_related_objects = [o for o in nests.RelatedObjects if o not in related_objects_set]
         if cur_related_objects:
             nests.RelatedObjects = list(cur_related_objects)
             ifcopenshell.api.run("owner.update_owner_history", file, **{"element": nests})
@@ -155,7 +155,11 @@ def assign_object(
 
     # assign elements to a new nesting
     if is_nested_by:
-        is_nested_by.RelatedObjects = list(set(is_nested_by.RelatedObjects) | related_objects)
+        cur_related_objects = list(is_nested_by.RelatedObjects)
+        cur_related_objects_set = set(cur_related_objects)
+        is_nested_by.RelatedObjects = cur_related_objects + [
+            o for o in related_objects if o not in cur_related_objects_set
+        ]
         ifcopenshell.api.run("owner.update_owner_history", file, **{"element": is_nested_by})
     else:
         is_nested_by = file.create_entity(
@@ -163,7 +167,7 @@ def assign_object(
             **{
                 "GlobalId": ifcopenshell.guid.new(),
                 "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", file),
-                "RelatedObjects": list(related_objects),
+                "RelatedObjects": related_objects,
                 "RelatingObject": relating_object,
             }
         )
diff --git a/src/ifcopenshell-python/ifcopenshell/api/nest/unassign_object.py b/src/ifcopenshell-python/ifcopenshell/api/nest/unassign_object.py
index 42e35777ad..bfd5549ad2 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/nest/unassign_object.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/nest/unassign_object.py
@@ -49,23 +49,23 @@ def unassign_object(file: ifcopenshell.file, related_objects: list[ifcopenshell.
         # nothing is returned, relationship is removed
         ifcopenshell.api.run("nest.unassign_object", model, related_objects=[subtask2])
     """
-    settings = {"related_objects": related_objects}
 
-    related_objects = set(settings["related_objects"])
+    # NOTE: maintain .RelatedObjects order as it has meaning in IFC
+    related_objects_set = set(related_objects)
     ifc2x3 = file.schema == "IFC2X3"
     if ifc2x3:
         rels = set(
             rel
-            for object in related_objects
+            for object in related_objects_set
             if (rel := next((rel for rel in object.Decomposes if rel.is_a("IfcRelNests")), None))
         )
     else:
         rels = set(rel for object in related_objects if (rel := next((rel for rel in object.Nests), None)))
 
     for rel in rels:
-        related_objects = set(rel.RelatedObjects) - related_objects
-        if related_objects:
-            rel.RelatedObjects = list(related_objects)
+        cur_related_objects = [o for o in rel.RelatedObjects if o not in related_objects_set]
+        if cur_related_objects:
+            rel.RelatedObjects = cur_related_objects
             ifcopenshell.api.run("owner.update_owner_history", file, **{"element": rel})
         else:
             history = rel.OwnerHistory
diff --git a/src/ifcopenshell-python/test/api/nest/test_assign_object.py b/src/ifcopenshell-python/test/api/nest/test_assign_object.py
index 25d4259627..181ff09fd7 100644
--- a/src/ifcopenshell-python/test/api/nest/test_assign_object.py
+++ b/src/ifcopenshell-python/test/api/nest/test_assign_object.py
@@ -66,6 +66,23 @@ class TestAssignObject(test.bootstrap.IFC4):
         with pytest.raises(RuntimeError):
             self.file.by_id(rel_id)
 
+    def test_maintain_assignment_order_in_related_objects(self):
+        element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTask")
+        subelements = [ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTask") for _ in range(5)]
+        for i in range(5):
+            ifcopenshell.api.run(
+                "nest.assign_object", self.file, related_objects=subelements[: i + 1], relating_object=element
+            )
+            rel = self.file.by_type("IfcRelNests")[0]
+            assert rel.RelatedObjects == tuple(subelements[: i + 1])
+
+        # maintain the order in the affected relationships too
+        element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTask")
+        ifcopenshell.api.run(
+            "nest.assign_object", self.file, related_objects=subelements[2:3], relating_object=element2
+        )
+        assert rel.RelatedObjects == tuple(subelements[:2] + subelements[3:])
+
 
 class TestAssignObjectIFC2X3(test.bootstrap.IFC2X3, TestAssignObject):
     pass
diff --git a/src/ifcopenshell-python/test/api/nest/test_unassign_object.py b/src/ifcopenshell-python/test/api/nest/test_unassign_object.py
index 479ab0bcce..a23c452adf 100644
--- a/src/ifcopenshell-python/test/api/nest/test_unassign_object.py
+++ b/src/ifcopenshell-python/test/api/nest/test_unassign_object.py
@@ -50,5 +50,21 @@ class TestUnassignObject(test.bootstrap.IFC4):
         ifcopenshell.api.run("nest.unassign_object", self.file, related_objects=[subelement])
         assert len(self.file.by_type("IfcRelNests")) == 0
 
+    def test_maintain_assignment_order_in_related_objects(self):
+        element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTask")
+        subelements = [ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcTask") for _ in range(5)]
+        rel = self.file.create_entity("IfcRelNests", RelatingObject=element, RelatedObjects=subelements)
+
+        original_order = subelements.copy()
+        # unassign elements in some random order
+        # skip 1 element to make sure rel won't get removed
+        removed_elements = set()
+        for i in (0, 3, 2, 4):
+            subelement = subelements[i]
+            ifcopenshell.api.run("nest.unassign_object", self.file, related_objects=[subelement])
+            removed_elements.add(subelement)
+            assert rel.RelatedObjects == tuple([o for o in original_order if o not in removed_elements])
+
+
 class TestUnassignObjectIFC2X3(test.bootstrap.IFC2X3, TestUnassignObject):
     pass