diff --git a/openff/toolkit/_tests/test_parameters.py b/openff/toolkit/_tests/test_parameters.py
index bd9ae0836..683cc30bd 100644
--- a/openff/toolkit/_tests/test_parameters.py
+++ b/openff/toolkit/_tests/test_parameters.py
@@ -822,6 +822,53 @@ class MyParameterHandler(ParameterHandler):
         ):
             handler.create_force()
 
+    def test_hash(self):
+        bh = BondHandler(skip_version_check=True, allow_cosmetic_attributes=True)
+
+        hash1 = hash(bh)
+        # Ensure hash changes when a new parameter is added
+        bh.add_parameter(
+            {
+                "smirks": "[*:1]-[*:2]",
+                "length": 1 * unit.angstrom,
+                "k": 10 * unit.kilocalorie / unit.mole / unit.angstrom ** 2,
+                "id": "b0",
+            }
+        )
+        hash2 = hash(bh)
+        assert hash1 != hash2
+
+        # Ensure hash changes when another parameter that differs only by SMIRKS is added
+        bh.add_parameter(
+            {
+                "smirks": "[C:1]-[C:2]",
+                "length": 1 * unit.angstrom,
+                "k": 10 * unit.kilocalorie / unit.mole / unit.angstrom ** 2,
+                "id": "b0",
+            }
+        )
+        hash3 = hash(bh)
+        assert hash2 != hash3
+
+        bh.add_cosmetic_attribute("fizz", "buzz")
+        hash3p5 = hash(bh)
+        assert hash3 != hash3p5
+
+        # Ensure hash changes when a cosmetic attribute is added
+        bh.parameters[0].add_cosmetic_attribute("foo", "bar")
+        hash4 = hash(bh)
+        assert hash3p5 != hash4
+
+        # Ensure hash changes when parameters are reordered
+        param = bh.parameters.pop(0)
+        bh.parameters.append(param)
+        hash5 = hash(bh)
+        assert hash4 != hash5
+
+        # Ensure hash doesn't change when the contents haven't changed
+        hash6 = hash(bh)
+        assert hash5 == hash6
+
 
 class TestParameterList:
     """Test capabilities of ParameterList for accessing and manipulating SMIRNOFF parameter definitions."""
diff --git a/openff/toolkit/typing/engines/smirnoff/parameters.py b/openff/toolkit/typing/engines/smirnoff/parameters.py
index b42fcba8b..d405c6e30 100644
--- a/openff/toolkit/typing/engines/smirnoff/parameters.py
+++ b/openff/toolkit/typing/engines/smirnoff/parameters.py
@@ -1905,6 +1905,38 @@ def __init__(
         # Initialize ParameterAttributes and cosmetic attributes.
         super().__init__(allow_cosmetic_attributes=allow_cosmetic_attributes, **kwargs)
 
+    def __hash__(self):
+        """
+        Hash a ParameterHandler and all of its contents (INCLUDING cosmetic attributes).
+
+        This method does not attempt to return the same hash for ParameterHandlers with equivalent
+        physics/chemistry but different cosmetic attributes or units. Instead this is a hash of all
+        of the ParameterHandler's contents, even if they don't affect system creation in any way.
+        """
+        handler_string = ''
+        attribute_dict = self.__dict__
+        for key, val in attribute_dict.items():
+            if isinstance(val, ParameterList):
+                handler_string += f'___{key}'
+                for parameter in val:
+                    # print(parameter)
+                    for attribute_name, attribute_val in parameter.__dict__.items():
+                        # print(attribute_name, attribute_val)
+                        if isinstance(attribute_val, list):
+                            #print(attribute_val)
+                            if len(attribute_val) > 0 and isinstance(attribute_val[0], Quantity):
+                                attribute_val = tuple([(i.m, hash(i.units)) for i in attribute_val])
+                            #else:
+                        if isinstance(attribute_val, Quantity):
+                            # print(dir(attribute_val))
+                            attribute_val = (attribute_val.m, hash(attribute_val.units))
+                            # break
+                        handler_string += f'__{attribute_name}_{attribute_val}'
+            else:
+                handler_string += f'{key}__{val}'
+
+        return hash(handler_string)
+
     def _add_parameters(self, section_dict, allow_cosmetic_attributes=False):
         """
         Extend the ParameterList in this ParameterHandler using a SMIRNOFF data source.