diff --git a/src/ifcopenshell-python/ifcopenshell/api/geometry/add_shape_aspect.py b/src/ifcopenshell-python/ifcopenshell/api/geometry/add_shape_aspect.py
new file mode 100644
index 0000000000..2f4ee941fe
--- /dev/null
+++ b/src/ifcopenshell-python/ifcopenshell/api/geometry/add_shape_aspect.py
@@ -0,0 +1,88 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 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 ifcopenshell
+from typing import Optional
+
+
+def add_shape_aspect(
+    file: ifcopenshell.file,
+    name: str,
+    items: list[ifcopenshell.entity_instance],
+    representation: ifcopenshell.entity_instance,
+    part_of_product: ifcopenshell.entity_instance,
+    description: Optional[str] = None,
+) -> ifcopenshell.entity_instance:
+    """Adds a shape aspect to items that are part of a representation and product
+
+    Existing shape aspects will be reused where possible. If the items already
+    belong to another shape aspect with a different name, this relationship
+    will be purged.
+
+    Warning: it is not possible to add a shape aspect to types (i.e.
+    IfcRepresentationMap) in IFC2X3.
+
+    :param name: The name of the shape aspect. This is case sensitive.
+    :param items: IfcRepresentationItems that will be assigned to this aspect.
+    :param representation: The IfcShapeRepresentation that the items are in.
+    :param part_of_product: The IfcRepresentationMap or
+        IfcProductDefinitionShape that the representation is in.
+    :param description: A description to set for the shape aspect. It's usually
+        not necessary.
+    :return: The IfcShapeAspect
+    """
+    result = None
+    items_set = set(items)
+    for aspect in part_of_product.HasShapeAspects or []:
+        if aspect.Name == name:
+            for aspect_rep in aspect.ShapeRepresentations:
+                if aspect_rep.ContextOfItems == representation.ContextOfItems:
+                    aspect.Description = description
+                    aspect_rep.Items = tuple(set(aspect_rep.Items) | items_set)
+                    result = aspect
+            if not result:
+                aspect_rep = file.createIfcShapeRepresentation(
+                    ContextOfItems=representation.ContextOfItems,
+                    RepresentationIdentifier=representation.RepresentationIdentifier,
+                    RepresentationType=representation.RepresentationType,
+                    Items=items,
+                )
+                aspect.ShapeRepresentations += (aspect_rep,)
+                result = aspect
+        else:
+            for aspect_rep in aspect.ShapeRepresentations:
+                if aspect_rep.ContextOfItems != representation.ContextOfItems:
+                    continue
+                if set(aspect_rep.Items) & items_set:
+                    if new_items := set(aspect_rep.Items) - items_set:
+                        aspect_rep.Items = tuple(new_items)
+                    else:
+                        file.remove(aspect_rep)
+            if not aspect.ShapeRepresentations:
+                file.remove(aspect)
+
+    if result:
+        return result
+
+    aspect_rep = file.createIfcShapeRepresentation(
+        ContextOfItems=representation.ContextOfItems,
+        RepresentationIdentifier=representation.RepresentationIdentifier,
+        RepresentationType=representation.RepresentationType,
+        Items=items,
+    )
+    return file.createIfcShapeAspect((aspect_rep,), name, description, True, part_of_product)
diff --git a/src/ifcopenshell-python/test/api/geometry/test_add_shape_aspect.py b/src/ifcopenshell-python/test/api/geometry/test_add_shape_aspect.py
new file mode 100644
index 0000000000..1ee1b3ac87
--- /dev/null
+++ b/src/ifcopenshell-python/test/api/geometry/test_add_shape_aspect.py
@@ -0,0 +1,161 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2025 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.root
+import ifcopenshell.api.context
+import ifcopenshell.api.geometry
+
+
+class TestAddShapeAspect(test.bootstrap.IFC4):
+    def test_adding_a_shape_aspect(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        model = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        body = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+        )
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        item = builder.sphere()
+        rep = builder.get_representation(body, [item])
+        element = ifcopenshell.api.root.create_entity(self.file)
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+
+        aspect = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item], representation=rep, part_of_product=element.Representation
+        )
+        assert aspect.is_a("IfcShapeAspect")
+        assert aspect.PartOfProductDefinitionShape == element.Representation
+        assert aspect.Name == "Foo"
+        assert len(aspect.ShapeRepresentations) == 1
+        aspect_rep = aspect.ShapeRepresentations[0]
+        assert aspect_rep != rep
+        assert aspect_rep.ContextOfItems == rep.ContextOfItems
+        assert aspect_rep.RepresentationIdentifier == rep.RepresentationIdentifier
+        assert aspect_rep.RepresentationType == rep.RepresentationType
+        assert aspect_rep.Items == (item,)
+
+    def test_adding_a_type_shape_aspect(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        model = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        body = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+        )
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        item = builder.sphere()
+        rep = builder.get_representation(body, [item])
+        element = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWallType")
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+
+        aspect = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item], representation=rep, part_of_product=element.RepresentationMaps[0]
+        )
+        assert aspect.is_a("IfcShapeAspect")
+        assert aspect.PartOfProductDefinitionShape == element.RepresentationMaps[0]
+        assert aspect.Name == "Foo"
+        assert len(aspect.ShapeRepresentations) == 1
+        aspect_rep = aspect.ShapeRepresentations[0]
+        assert aspect_rep != rep
+        assert aspect_rep.ContextOfItems == rep.ContextOfItems
+        assert aspect_rep.RepresentationIdentifier == rep.RepresentationIdentifier
+        assert aspect_rep.RepresentationType == rep.RepresentationType
+        assert aspect_rep.Items == (item,)
+
+    def test_reusing_an_existing_aspect(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        model = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        body = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+        )
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        item = builder.sphere()
+        item2 = builder.sphere()
+        rep = builder.get_representation(body, [item, item2])
+        element = ifcopenshell.api.root.create_entity(self.file)
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+
+        aspect = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item], representation=rep, part_of_product=element.Representation
+        )
+        aspect2 = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item2], representation=rep, part_of_product=element.Representation
+        )
+        assert aspect == aspect2
+        assert len(aspect.ShapeRepresentations) == 1
+        assert set(aspect.ShapeRepresentations[0].Items) == {item, item2}
+
+    def test_removing_from_previous_aspects(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        model = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        body = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+        )
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        item = builder.sphere()
+        item2 = builder.sphere()
+        rep = builder.get_representation(body, [item, item2])
+        element = ifcopenshell.api.root.create_entity(self.file)
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+
+        aspect = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item, item2], representation=rep, part_of_product=element.Representation
+        )
+        aspect2 = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Bar", items=[item2], representation=rep, part_of_product=element.Representation
+        )
+        assert aspect != aspect2
+        assert aspect.Name == "Foo"
+        assert aspect.ShapeRepresentations[0].Items == (item,)
+        assert aspect2.Name == "Bar"
+        assert aspect2.ShapeRepresentations[0].Items == (item2,)
+
+    def test_take_context_into_account(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        model = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        body = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+        )
+        box = ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Box", target_view="MODEL_VIEW", parent=model
+        )
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        item = builder.sphere()
+        item2 = builder.sphere()
+        rep = builder.get_representation(body, [item, item2])
+        rep2 = builder.get_representation(box, [item, item2])
+        element = ifcopenshell.api.root.create_entity(self.file)
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep)
+        ifcopenshell.api.geometry.assign_representation(self.file, product=element, representation=rep2)
+
+        aspect = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item, item2], representation=rep, part_of_product=element.Representation
+        )
+        aspect2 = ifcopenshell.api.geometry.add_shape_aspect(
+            self.file, name="Foo", items=[item2], representation=rep2, part_of_product=element.Representation
+        )
+        assert aspect == aspect2
+        assert aspect.Name == "Foo"
+        assert len(aspect.ShapeRepresentations) == 2
+        assert aspect.ShapeRepresentations[0].ContextOfItems == rep.ContextOfItems
+        assert aspect.ShapeRepresentations[1].ContextOfItems == rep2.ContextOfItems
+        assert set(aspect.ShapeRepresentations[0].Items) == {item, item2}
+        assert aspect.ShapeRepresentations[1].Items == (item2,)
+
+
+class TestAddShapeAspectIFC2X3(test.bootstrap.IFC2X3, TestAddShapeAspect):
+    def test_adding_a_type_shape_aspect(self):
+        pass  # Not allowed