diff --git a/src/blenderbim/blenderbim/bim/module/project/operator.py b/src/blenderbim/blenderbim/bim/module/project/operator.py
index 68c562c1c0..11b817c77f 100644
--- a/src/blenderbim/blenderbim/bim/module/project/operator.py
+++ b/src/blenderbim/blenderbim/bim/module/project/operator.py
@@ -338,7 +338,7 @@ class UnassignLibraryDeclaration(bpy.types.Operator):
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.file.by_id(self.definition),
+            definitions=[self.file.by_id(self.definition)],
             relating_context=self.file.by_type("IfcProjectLibrary")[0],
         )
         element_name = self.props.active_library_element
diff --git a/src/ifcopenshell-python/ifcopenshell/api/__init__.py b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
index 1d21eaf40f..4249ed96aa 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/__init__.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
@@ -120,6 +120,9 @@ ARGUMENTS_DEPRECATION = {
     "project.assign_declaration": partial(
         batching_argument_deprecation, prev_argument="definition", new_argument="definitions"
     ),
+    "project.unassign_declaration": partial(
+        batching_argument_deprecation, prev_argument="definition", new_argument="definitions"
+    ),
 }
 
 
diff --git a/src/ifcopenshell-python/ifcopenshell/api/project/unassign_declaration.py b/src/ifcopenshell-python/ifcopenshell/api/project/unassign_declaration.py
index 7e93f558fc..47c5194bdd 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/project/unassign_declaration.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/project/unassign_declaration.py
@@ -22,13 +22,19 @@ import ifcopenshell.util.element
 
 
 class Usecase:
-    def __init__(self, file, definition=None, relating_context=None):
-        """Unassigns an object to a project or project library
+    def __init__(
+        self,
+        file: ifcopenshell.file,
+        definitions: list[ifcopenshell.entity_instance],
+        relating_context: ifcopenshell.entity_instance,
+    ):
+        """Unassigns a list of objects from a project or project library
 
         Typically used to remove an asset from a project library.
 
-        :param definition: The object you want to undeclare. Typically an asset.
-        :type definition: ifcopenshell.entity_instance.entity_instance
+        :param definitions: The list of objects you want to undeclare.
+            Typically a list of assets.
+        :type definitions: list[ifcopenshell.entity_instance.entity_instance]
         :param relating_context: The IfcProject, or more commonly the
             IfcProjectLibrary that you want the object to no longer be part of.
         :type relating_context: ifcopenshell.entity_instance.entity_instance
@@ -49,25 +55,25 @@ class Usecase:
             ifcopenshell.api.run("project.assign_declaration", library, definitions=[context], relating_context=root)
 
             # Remove the library from our project
-            ifcopenshell.api.run("project.unassign_declaration", library, definition=context, relating_context=root)
+            ifcopenshell.api.run("project.unassign_declaration", library, definitions=[context], relating_context=root)
         """
         self.file = file
         self.settings = {
-            "definition": definition,
+            "definitions": definitions,
             "relating_context": relating_context,
         }
 
     def execute(self):
-        if not self.settings["definition"].HasContext:
-            return
-        rel = self.settings["definition"].HasContext[0]
-        related_definitions = set(rel.RelatedDefinitions) or set()
-        related_definitions.remove(self.settings["definition"])
-        if len(related_definitions):
-            rel.RelatedDefinitions = list(related_definitions)
-            ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": rel})
-        else:
-            history = rel.OwnerHistory
-            self.file.remove(rel)
-            if history:
-                ifcopenshell.util.element.remove_deep2(self.file, history)
+        definitions = set(self.settings["definitions"])
+        rels = {rel for obj in definitions if (rel := next(iter(obj.HasContext), None))}
+
+        for rel in rels:
+            related_definitions = set(rel.RelatedDefinitions) - definitions
+            if related_definitions:
+                rel.RelatedDefinitions = list(related_definitions)
+                ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": rel})
+            else:
+                history = rel.OwnerHistory
+                self.file.remove(rel)
+                if history:
+                    ifcopenshell.util.element.remove_deep2(self.file, history)
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
index a9e6f000eb..b3573db5d5 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
@@ -57,7 +57,7 @@ class Usecase:
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.settings["work_schedule"],
+            definitions=[self.settings["work_schedule"]],
             relating_context=self.file.by_type("IfcContext")[0],
         )
         rel_aggregates = ifcopenshell.api.run(
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_task.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_task.py
index 4c0ee504f4..870efa1876 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_task.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_task.py
@@ -62,7 +62,7 @@ class Usecase:
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.settings["task"],
+            definitions=[self.settings["task"]],
             relating_context=self.file.by_type("IfcContext")[0],
         )
         if self.settings["task"].TaskTime:
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_calendar.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_calendar.py
index a630c3f42e..242165c201 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_calendar.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_calendar.py
@@ -50,7 +50,7 @@ class Usecase:
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.settings["work_calendar"],
+            definitions=[self.settings["work_calendar"]],
             relating_context=self.file.by_type("IfcContext")[0],
         )
         if self.settings["work_calendar"].Controls:
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_plan.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_plan.py
index cb8c638799..905a7b9b8a 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_plan.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_plan.py
@@ -50,7 +50,7 @@ class Usecase:
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.settings["work_plan"],
+            definitions=[self.settings["work_plan"]],
             relating_context=self.file.by_type("IfcContext")[0],
         )
         history = self.settings["work_plan"].OwnerHistory
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_schedule.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_schedule.py
index 935c9f1832..b8bbcd4616 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_schedule.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/remove_work_schedule.py
@@ -54,7 +54,7 @@ class Usecase:
         ifcopenshell.api.run(
             "project.unassign_declaration",
             self.file,
-            definition=self.settings["work_schedule"],
+            definitions=[self.settings["work_schedule"]],
             relating_context=self.file.by_type("IfcContext")[0],
         )
         if self.settings["work_schedule"].Declares:
diff --git a/src/ifcopenshell-python/test/api/project/test_unassign_declaration.py b/src/ifcopenshell-python/test/api/project/test_unassign_declaration.py
new file mode 100644
index 0000000000..60dd46f7ad
--- /dev/null
+++ b/src/ifcopenshell-python/test/api/project/test_unassign_declaration.py
@@ -0,0 +1,82 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2021 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+import test.bootstrap
+import ifcopenshell.api
+from typing import Union
+
+
+# NOTE: supported only in IFC4+
+class TestUnassignDeclaration(test.bootstrap.IFC4):
+    def get_context(self, definition: ifcopenshell.entity_instance) -> Union[ifcopenshell.entity_instance, None]:
+        rel = next(iter(definition.HasContext), None)
+        if rel is not None:
+            return rel.RelatingContext
+
+    def test_unassigning_a_definition(self):
+        library = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProjectLibrary")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.run(
+            "project.assign_declaration", self.file, definitions=[element_type, element_type2], relating_context=library
+        )
+        ifcopenshell.api.run(
+            "project.unassign_declaration",
+            self.file,
+            definitions=[element_type, element_type2],
+            relating_context=library,
+        )
+        assert self.get_context(element_type) == None
+        assert len(self.file.by_type("IfcRelDeclares")) == 0
+
+    def test_doing_nothing_if_there_was_no_declaration(self):
+        library = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProjectLibrary")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.run(
+            "project.unassign_declaration",
+            self.file,
+            definitions=[element_type, element_type2],
+            relating_context=library,
+        )
+        assert self.get_context(element_type) == None
+        assert self.get_context(element_type2) == None
+
+    def test_updating_the_rel_when_a_reference_is_removed_with_multipled_elements(self):
+        library = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProjectLibrary")
+        element_type1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        element_type3 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.run(
+            "project.assign_declaration", self.file, definitions=[element_type1], relating_context=library
+        )
+        rel = self.file.by_type("IfcRelDeclares")[0]
+
+        ifcopenshell.api.run(
+            "project.assign_declaration",
+            self.file,
+            definitions=[element_type2, element_type3],
+            relating_context=library,
+        )
+        ifcopenshell.api.run(
+            "project.unassign_declaration",
+            self.file,
+            definitions=[element_type1, element_type2],
+            relating_context=library,
+        )
+        assert rel.RelatedDefinitions == (element_type3,)
diff --git a/src/ifcopenshell-python/test/api/test_api.py b/src/ifcopenshell-python/test/api/test_api.py
index 2f4db8786f..00d33b8b8a 100644
--- a/src/ifcopenshell-python/test/api/test_api.py
+++ b/src/ifcopenshell-python/test/api/test_api.py
@@ -23,6 +23,7 @@ import ifcopenshell.util.constraint
 import ifcopenshell.util.element
 import ifcopenshell.util.system
 from datetime import datetime
+from typing import Union
 
 
 def deprecation_check(test):
@@ -326,3 +327,24 @@ class TestTemporarySupportForDeprecatedAPIArguments(test.bootstrap.IFC4):
         )
         assert get_declared_definitions(library) == {element_type}
         assert len(self.file.by_type("IfcRelDeclares")) == 1
+
+    @deprecation_check
+    def test_unassigning_a_definition(self):
+        def get_context(definition: ifcopenshell.entity_instance) -> Union[ifcopenshell.entity_instance, None]:
+            rel = next(iter(definition.HasContext), None)
+            if rel is not None:
+                return rel.RelatingContext
+
+        library = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProjectLibrary")
+        element_type = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.run(
+            "project.assign_declaration", self.file, definitions=[element_type], relating_context=library
+        )
+        ifcopenshell.api.run(
+            "project.unassign_declaration",
+            self.file,
+            definition=element_type,
+            relating_context=library,
+        )
+        assert get_context(element_type) == None
+        assert len(self.file.by_type("IfcRelDeclares")) == 0