diff --git a/fuzz/fuzz_seeds/postgres_alter_extension.sql b/fuzz/fuzz_seeds/postgres_alter_extension.sql
index 20931ac0fd..aeb19f1ff8 100644
--- a/fuzz/fuzz_seeds/postgres_alter_extension.sql
+++ b/fuzz/fuzz_seeds/postgres_alter_extension.sql
@@ -34,3 +34,4 @@ ALTER EXTENSION citext ADD AGGREGATE cnt(*);
 ALTER EXTENSION citext ADD OPERATOR - (NONE, int);
 ALTER EXTENSION citext DROP OPERATOR CLASS oc USING btree;
 ALTER EXTENSION citext DROP OPERATOR FAMILY of USING btree;
+ALTER EXTENSION citext ADD CAST (int AS text);
diff --git a/src/ast/ddl.rs b/src/ast/ddl.rs
index 01d6c392c8..074f8f55e5 100644
--- a/src/ast/ddl.rs
+++ b/src/ast/ddl.rs
@@ -4934,6 +4934,13 @@ pub enum AlterExtensionMember {
     AccessMethod(Ident),
     /// `AGGREGATE aggregate_name ( aggregate_signature )`
     Aggregate(AggregateSignature),
+    /// `CAST (source_type AS target_type)`
+    Cast {
+        /// Source type.
+        source_type: DataType,
+        /// Target type.
+        target_type: DataType,
+    },
     /// `COLLATION object_name`
     Collation(ObjectName),
     /// `CONVERSION object_name`
@@ -5035,6 +5042,10 @@ impl fmt::Display for AlterExtensionMember {
         match self {
             AlterExtensionMember::AccessMethod(name) => write!(f, "ACCESS METHOD {name}"),
             AlterExtensionMember::Aggregate(signature) => write!(f, "AGGREGATE {signature}"),
+            AlterExtensionMember::Cast {
+                source_type,
+                target_type,
+            } => write!(f, "CAST ({source_type} AS {target_type})"),
             AlterExtensionMember::Collation(name) => write!(f, "COLLATION {name}"),
             AlterExtensionMember::Conversion(name) => write!(f, "CONVERSION {name}"),
             AlterExtensionMember::Database(name) => write!(f, "DATABASE {name}"),
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index cd78d41dbc..19e9444594 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -12536,6 +12536,16 @@ impl<'a> Parser<'a> {
             Ok(AlterExtensionMember::Aggregate(
                 self.parse_alter_aggregate_signature()?,
             ))
+        } else if self.parse_keyword(Keyword::CAST) {
+            self.expect_token(&Token::LParen)?;
+            let source_type = self.parse_data_type()?;
+            self.expect_keyword_is(Keyword::AS)?;
+            let target_type = self.parse_data_type()?;
+            self.expect_token(&Token::RParen)?;
+            Ok(AlterExtensionMember::Cast {
+                source_type,
+                target_type,
+            })
         } else if self.parse_keyword(Keyword::COLLATION) {
             Ok(AlterExtensionMember::Collation(
                 self.parse_object_name(false)?,
diff --git a/tests/sqlparser_postgres.rs b/tests/sqlparser_postgres.rs
index dc86429746..3d816c7af1 100644
--- a/tests/sqlparser_postgres.rs
+++ b/tests/sqlparser_postgres.rs
@@ -13541,6 +13541,8 @@ fn alter_extension_member_round_trip_forms(action: &str) {
         format!("ALTER EXTENSION ext_name {action} AGGREGATE agg(IN INT)"),
         format!("ALTER EXTENSION ext_name {action} AGGREGATE agg(ORDER BY TEXT)"),
         format!("ALTER EXTENSION ext_name {action} AGGREGATE agg(INT ORDER BY TEXT)"),
+        format!("ALTER EXTENSION ext_name {action} CAST (INT AS TEXT)"),
+        format!("ALTER EXTENSION ext_name {action} CAST (src AS tgt)"),
         format!("ALTER EXTENSION ext_name {action} COLLATION en_US"),
         format!("ALTER EXTENSION ext_name {action} COLLATION schema.en_US"),
         format!("ALTER EXTENSION ext_name {action} CONVERSION conv"),
@@ -13739,6 +13741,27 @@ fn parse_alter_extension_add_member() {
         })
     );
 
+    match pg_and_generic().verified_stmt("ALTER EXTENSION ext_name ADD CAST (src AS tgt)") {
+        Statement::AlterExtension(AlterExtension {
+            operation:
+                AlterExtensionOperation::Add(AlterExtensionMember::Cast {
+                    source_type,
+                    target_type,
+                }),
+            ..
+        }) => {
+            assert_eq!(
+                source_type,
+                DataType::Custom(ObjectName::from(Ident::new("src")), vec![])
+            );
+            assert_eq!(
+                target_type,
+                DataType::Custom(ObjectName::from(Ident::new("tgt")), vec![])
+            );
+        }
+        other => panic!("unexpected statement: {other:?}"),
+    }
+
     match pg_and_generic().verified_stmt("ALTER EXTENSION ext_name ADD FUNCTION f(INT)") {
         Statement::AlterExtension(AlterExtension {
             operation: AlterExtensionOperation::Add(AlterExtensionMember::Function(function)),
@@ -13807,6 +13830,10 @@ fn parse_alter_extension_add_member() {
             "ALTER EXTENSION ext_name ADD ROUTINE 1",
             "Expected: identifier, found: 1",
         ),
+        (
+            "ALTER EXTENSION ext_name ADD CAST (INT)",
+            "Expected: AS, found: )",
+        ),
         (
             "ALTER EXTENSION ext_name ADD OPERATOR +",
             "Expected: (, found: EOF",
@@ -13819,6 +13846,22 @@ fn parse_alter_extension_add_member() {
             "ALTER EXTENSION ext_name ADD OPERATOR CLASS oc USING",
             "Expected: identifier, found: EOF",
         ),
+        (
+            "ALTER EXTENSION ext_name ADD CAST",
+            "Expected: (, found: EOF",
+        ),
+        (
+            "ALTER EXTENSION ext_name ADD CAST (1)",
+            "Expected: a data type name, found: 1",
+        ),
+        (
+            "ALTER EXTENSION ext_name ADD CAST (INT AS)",
+            "Expected: a data type name, found: )",
+        ),
+        (
+            "ALTER EXTENSION ext_name ADD CAST (INT AS INT",
+            "Expected: ), found: EOF",
+        ),
         (
             "ALTER EXTENSION ext_name ADD OPERATOR CLASS",
             "Expected: identifier, found: EOF",