diff --git a/src/plumpy/processes.py b/src/plumpy/processes.py
index 41bd3609..5c8326b3 100644
--- a/src/plumpy/processes.py
+++ b/src/plumpy/processes.py
@@ -731,8 +731,18 @@ def on_create(self) -> None:
         """Entering the CREATED state."""
         self._creation_time = time.time()
 
-        # This will parse the inputs with respect to the input portnamespace of the spec and validate them
-        raw_inputs = dict(self._raw_inputs) if self._raw_inputs else {}
+        def recursively_copy_dictionaries(value):
+            """Recursively copy the mapping but only create copies of the dictionaries not the values."""
+            if isinstance(value, dict):
+                return {key: recursively_copy_dictionaries(subvalue) for key, subvalue in value.items()}
+            return value
+
+        # This will parse the inputs with respect to the input portnamespace of the spec and validate them. The
+        # ``pre_process`` method of the inputs port namespace modifies its argument in place, and since the
+        # ``_raw_inputs`` should not be modified, we pass a clone of it. Note that we only need a clone of the nested
+        # dictionaries, so we don't use ``copy.deepcopy`` (which might seem like the obvious choice) as that will also
+        # create a clone of the values, which we don't want.
+        raw_inputs = recursively_copy_dictionaries(dict(self._raw_inputs)) if self._raw_inputs else {}
         self._parsed_inputs = self.spec().inputs.pre_process(raw_inputs)
         result = self.spec().inputs.validate(self._parsed_inputs)
 
diff --git a/test/test_processes.py b/test/test_processes.py
index e357fc4d..d0868e49 100644
--- a/test/test_processes.py
+++ b/test/test_processes.py
@@ -115,6 +115,28 @@ def define(cls, spec):
         with self.assertRaises(AttributeError):
             p.raw_inputs.b
 
+    def test_raw_inputs(self):
+        """Test that the ``raw_inputs`` are not mutated by the ``Process`` constructor.
+
+        Regression test for https://github.com/aiidateam/plumpy/issues/250
+        """
+
+        class Proc(Process):
+
+            @classmethod
+            def define(cls, spec):
+                super().define(spec)
+                spec.input('a')
+                spec.input('nested.a')
+                spec.input('nested.b', default='default-value')
+
+        inputs = {'a': 5, 'nested': {'a': 'value'}}
+        process = Proc(inputs)
+
+        # Compare against a clone of the original inputs dictionary as the original is modified. It should not contain
+        # the default value of the ``nested.b`` port.
+        self.assertDictEqual(dict(process.raw_inputs), {'a': 5, 'nested': {'a': 'value'}})
+
     def test_inputs_default(self):
 
         class Proc(utils.DummyProcess):