diff --git a/openmc/material.py b/openmc/material.py
index c7b954b6666..5922c044717 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -524,7 +524,10 @@ def set_density(self, units: str, density: float | None = None):
             self._density = density
 
     def add_nuclide(self, nuclide: str, percent: float, percent_type: str = 'ao'):
-        """Add a nuclide to the material
+        """Add a nuclide to the material.
+
+        If nuclide with the same name and percent_type already exists in the
+        material, its percentage will be updated.
 
         Parameters
         ----------
@@ -559,7 +562,24 @@ def add_nuclide(self, nuclide: str, percent: float, percent_type: str = 'ao'):
             if Z >= 89:
                 self.depletable = True
 
-        self._nuclides.append(NuclideTuple(nuclide, percent, percent_type))
+        # Flag to mark if we squashed the nuclide with an existing one
+        squashed = False
+
+        for i, nt in enumerate(self._nuclides):
+            if nt.name == nuclide and nt.percent_type == percent_type:
+                # add the percentage the two matching nuclides
+                self._nuclides[i] = NuclideTuple(nuclide, nt.percent + percent, percent_type)
+                squashed = True
+                break
+            elif nt.name == nuclide and nt.percent_type != percent_type:
+                warnings.warn(
+                    f"Nuclide '{nuclide}' already present with percent_type "
+                    f"'{nt.percent_type}'. Keeping separate entry for "
+                    f"new percent_type '{percent_type}'."
+                )
+
+        if not squashed:
+            self._nuclides.append(NuclideTuple(nuclide, percent, percent_type))
 
     def add_components(self, components: dict, percent_type: str = 'ao'):
         """ Add multiple elements or nuclides to a material
diff --git a/tests/unit_tests/test_material.py b/tests/unit_tests/test_material.py
index eae814a755b..3de0f88d49f 100644
--- a/tests/unit_tests/test_material.py
+++ b/tests/unit_tests/test_material.py
@@ -93,15 +93,19 @@ def test_nuclides_to_ignore(run_in_tmpdir):
 def test_remove_nuclide():
     """Test removing nuclides."""
     m = openmc.Material()
-    for nuc, percent in [('H1', 1.0), ('H2', 1.0), ('H1', 2.0), ('H2', 2.0)]:
-        m.add_nuclide(nuc, percent)
+    for nuc, percent, percent_type in [
+        ('H1', 1.0, 'ao'),
+        ('H2', 1.0, 'ao'),
+        ('H1', 2.0, 'wo'),
+        ('H2', 2.0, 'wo')
+    ]:
+        m.add_nuclide(nuc, percent=percent, percent_type=percent_type)
     m.remove_nuclide('H1')
     assert len(m.nuclides) == 2
     assert all(nuc.name == 'H2' for nuc in m.nuclides)
     assert m.nuclides[0].percent == 1.0
     assert m.nuclides[1].percent == 2.0
 
-
 def test_remove_elements():
     """Test removing elements."""
     m = openmc.Material()
@@ -766,3 +770,34 @@ def test_mean_free_path():
     mat2.add_nuclide('Pb208', 1.0)
     mat2.set_density('g/cm3', 11.34)
     assert mat2.mean_free_path(energy=14e6) == pytest.approx(5.65, abs=1e-2)
+
+
+def test_add_nuclide_squash_same_type():
+    mat1 = openmc.Material()
+    mat1.add_nuclide('Li6', 0.02, 'ao')
+    mat1.add_nuclide('Li6', 0.03, 'ao')
+    # Should be a single entry with 0.05 ao
+    entries = [nt for nt in mat1.nuclides if nt.name == 'Li6' and nt.percent_type == 'ao']
+    assert len(entries) == 1
+    assert entries[0].percent == pytest.approx(0.05)
+
+    mat2 = openmc.Material()
+    mat2.add_nuclide('Li6', 0.04, 'wo')
+    mat2.add_nuclide('Li6', 0.05, 'wo')
+    # Should be a single entry with 0.09 wo
+    entries = [nt for nt in mat2.nuclides if nt.name == 'Li6' and nt.percent_type == 'wo']
+    assert len(entries) == 1
+    assert entries[0].percent == pytest.approx(0.09)
+
+
+def test_add_nuclide_keep_different_type_warns():
+    mat = openmc.Material()
+    mat.add_nuclide('Li6', 0.02, 'ao')
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("always")
+        mat.add_nuclide('Li6', 0.01, 'wo')
+        assert any("Keeping separate entry" in str(rec.message) for rec in w)
+    # Should have two entries: one ao and one wo
+    aos = [nt for nt in mat.nuclides if nt.name == 'Li6' and nt.percent_type == 'ao']
+    wos = [nt for nt in mat.nuclides if nt.name == 'Li6' and nt.percent_type == 'wo']
+    assert len(aos) == 1 and len(wos) == 1
\ No newline at end of file