diff --git a/CHANGES.txt b/CHANGES.txt
index 5f08069bda..812e02051a 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,6 +1,7 @@
 3.11.8
  * Frozen RawTuple is not annotated with frozen in the toString method (CASSANDRA-15857)
 Merged from 3.0:
+ * Forbid altering UDTs used in partition keys (CASSANDRA-15933)
  * Fix empty/null json string representation (CASSANDRA-15896)
 Merged from 2.2:
  * Fix CQL parsing of collections when the column type is reversed (CASSANDRA-15814)
diff --git a/src/java/org/apache/cassandra/cql3/statements/AlterTypeStatement.java b/src/java/org/apache/cassandra/cql3/statements/AlterTypeStatement.java
index 71d19fa37e..d0e2f33979 100644
--- a/src/java/org/apache/cassandra/cql3/statements/AlterTypeStatement.java
+++ b/src/java/org/apache/cassandra/cql3/statements/AlterTypeStatement.java
@@ -31,6 +31,10 @@ import org.apache.cassandra.service.MigrationManager;
 import org.apache.cassandra.service.QueryState;
 import org.apache.cassandra.transport.Event;
 
+import static com.google.common.collect.Iterables.any;
+import static com.google.common.collect.Iterables.filter;
+import static com.google.common.collect.Iterables.transform;
+
 public abstract class AlterTypeStatement extends SchemaAlteringStatement
 {
     protected final UTName name;
@@ -262,12 +266,31 @@ public abstract class AlterTypeStatement extends SchemaAlteringStatement
             if (addType.referencesUserType(toUpdate.getNameAsString()))
                 throw new InvalidRequestException(String.format("Cannot add new field %s of type %s to type %s as this would create a circular reference", fieldName, type, name));
 
+            Collection<CFMetaData> tablesWithTypeInPartitionKey = findTablesReferencingTypeInPartitionKey(ksm, name.getStringTypeName());
+            if (!tablesWithTypeInPartitionKey.isEmpty())
+            {
+                String message =
+                    String.format("Cannot add new field %s of type %s to user type %s.%s as the type is being used in partition key by the following tables: %s",
+                                  fieldName, type, ksm.name, toUpdate.getNameAsString(),
+                                  String.join(", ", transform(tablesWithTypeInPartitionKey, table -> table.ksName + '.' + table.cfName)));
+                throw new InvalidRequestException(message);
+            }
+
             List<AbstractType<?>> newTypes = new ArrayList<>(toUpdate.size() + 1);
             newTypes.addAll(toUpdate.fieldTypes());
             newTypes.add(addType);
 
             return new UserType(toUpdate.keyspace, toUpdate.name, newNames, newTypes, toUpdate.isMultiCell());
         }
+
+        private static Collection<CFMetaData> findTablesReferencingTypeInPartitionKey(KeyspaceMetadata keyspace, String userTypeName)
+        {
+            Collection<CFMetaData> tables = new ArrayList<>();
+            filter(keyspace.tablesAndViews(),
+                   table -> any(table.partitionKeyColumns(), column -> column.type.referencesUserType(userTypeName)))
+                  .forEach(tables::add);
+            return tables;
+        }
     }
 
     private static class Renames extends AlterTypeStatement
diff --git a/test/unit/org/apache/cassandra/cql3/validation/operations/AlterTest.java b/test/unit/org/apache/cassandra/cql3/validation/operations/AlterTest.java
index 238dd072b8..1d6b064086 100644
--- a/test/unit/org/apache/cassandra/cql3/validation/operations/AlterTest.java
+++ b/test/unit/org/apache/cassandra/cql3/validation/operations/AlterTest.java
@@ -547,4 +547,23 @@ public class AlterTest extends CQLTester
         execute("ALTER TABLE %s ALTER value1 TYPE 'org.apache.cassandra.db.marshal.BytesType'");
     }
 
+    @Test
+    public void testAlterTypeUsedInPartitionKey() throws Throwable
+    {
+        // frozen UDT used directly in a partition key
+        String  type1 = createType("CREATE TYPE %s (v1 int)");
+        String table1 = createTable("CREATE TABLE %s (pk frozen<" + type1 + ">, val int, PRIMARY KEY(pk));");
+
+        // frozen UDT used in a frozen UDT used in a partition key
+        String  type2 = createType("CREATE TYPE %s (v1 frozen<" + type1 + ">, v2 frozen<" + type1 + ">)");
+        String table2 = createTable("CREATE TABLE %s (pk frozen<" + type2 + ">, val int, PRIMARY KEY(pk));");
+
+        // frozen UDT used in a frozen collection used in a partition key
+        String table3 = createTable("CREATE TABLE %s (pk frozen<list<frozen<" + type1 + ">>>, val int, PRIMARY KEY(pk));");
+
+        // assert that ALTER fails and that the error message contains all the names of the table referencing it
+        assertInvalidMessage(table1, format("ALTER TYPE %s.%s ADD v2 int;", keyspace(), type1));
+        assertInvalidMessage(table2, format("ALTER TYPE %s.%s ADD v2 int;", keyspace(), type1));
+        assertInvalidMessage(table3, format("ALTER TYPE %s.%s ADD v2 int;", keyspace(), type1));
+    }
 }