diff --git a/mysql-test/r/innodb.result b/mysql-test/r/innodb.result
index de2fbb3c295..bbeaf3519c2 100644
--- a/mysql-test/r/innodb.result
+++ b/mysql-test/r/innodb.result
@@ -3133,9 +3133,7 @@ SHOW CREATE TABLE t2;
 Table	Create Table
 t2	CREATE TABLE `t2` (
   `a` int(11) DEFAULT NULL,
-  KEY `t2_ibfk_0` (`a`),
-  CONSTRAINT `t2_ibfk_0` FOREIGN KEY (`a`) REFERENCES `t1` (`a`),
-  CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`a`) REFERENCES `t1` (`a`)
+  KEY `t2_ibfk_0` (`a`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
 DROP TABLE t2,t1;
 create table t1(a int not null, b int, c int, d int, primary key(a)) engine=innodb;
@@ -3214,3 +3212,34 @@ UPDATE t1 SET field1 = 'other' WHERE field2 = 'somevalu';
 ERROR 23000: Upholding foreign key constraints for table 't1', entry 'other-somevalu', key 1 would lead to a duplicate entry
 DROP TABLE t2;
 DROP TABLE t1;
+create table t1 (
+c1 bigint not null,
+c2 bigint not null,
+primary key (c1),
+unique  key (c2)
+) engine=innodb;
+create table t2 (
+c1 bigint not null,
+primary key (c1)
+) engine=innodb;
+alter table t1 add constraint c2_fk foreign key (c2)
+references t2(c1) on delete cascade;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c1` bigint(20) NOT NULL,
+  `c2` bigint(20) NOT NULL,
+  PRIMARY KEY (`c1`),
+  UNIQUE KEY `c2` (`c2`),
+  CONSTRAINT `c2_fk` FOREIGN KEY (`c2`) REFERENCES `t2` (`c1`) ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+alter table t1 drop foreign key c2_fk;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `c1` bigint(20) NOT NULL,
+  `c2` bigint(20) NOT NULL,
+  PRIMARY KEY (`c1`),
+  UNIQUE KEY `c2` (`c2`)
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+drop table t1, t2;
diff --git a/mysql-test/t/innodb.test b/mysql-test/t/innodb.test
index 27d9814404d..0a8e46d694c 100644
--- a/mysql-test/t/innodb.test
+++ b/mysql-test/t/innodb.test
@@ -2113,3 +2113,28 @@ UPDATE t1 SET field1 = 'other' WHERE field2 = 'somevalu';
 
 DROP TABLE t2;
 DROP TABLE t1;
+
+#
+# Bug#18477 - MySQL/InnoDB Ignoring Foreign Keys in ALTER TABLE
+#
+create table t1 (
+  c1 bigint not null,
+  c2 bigint not null,
+  primary key (c1),
+  unique  key (c2)
+) engine=innodb;
+#
+create table t2 (
+  c1 bigint not null,
+  primary key (c1)
+) engine=innodb;
+#
+alter table t1 add constraint c2_fk foreign key (c2)
+  references t2(c1) on delete cascade;
+show create table t1;
+#
+alter table t1 drop foreign key c2_fk;
+show create table t1;
+#
+drop table t1, t2;
+
diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index 1fea4971c4d..f5141806b38 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -706,6 +706,7 @@ typedef class st_select_lex SELECT_LEX;
 #define ALTER_CHECK_PARTITION    (1L << 23)
 #define ALTER_REPAIR_PARTITION   (1L << 24)
 #define ALTER_REMOVE_PARTITIONING (1L << 25)
+#define ALTER_FOREIGN_KEY         (1L << 26)
 
 typedef struct st_alter_info
 {
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index ed875d321bc..2687b64841f 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -3796,7 +3796,7 @@ static uint compare_tables(TABLE *table, List<create_field> *create_list,
       create_info->used_fields & HA_CREATE_USED_ENGINE ||
       create_info->used_fields & HA_CREATE_USED_CHARSET ||
       create_info->used_fields & HA_CREATE_USED_DEFAULT_CHARSET ||
-      (alter_info->flags & ALTER_RECREATE) ||
+      (alter_info->flags & (ALTER_RECREATE | ALTER_FOREIGN_KEY)) ||
       order_num)
     DBUG_RETURN(ALTER_TABLE_DATA_CHANGED);
 
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index e9379ab2f18..28592541c98 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -4167,6 +4167,9 @@ key_def:
 					    HA_KEY_ALG_UNDEF, 1,
 					    lex->col_list));
 	    lex->col_list.empty();		/* Alloced by sql_alloc */
+
+            /* Only used for ALTER TABLE. Ignored otherwise. */
+            lex->alter_info.flags|= ALTER_FOREIGN_KEY;
 	  }
 	| constraint opt_check_constraint
 	  {
@@ -5137,7 +5140,7 @@ alter_list_item:
 	  }
 	| DROP FOREIGN KEY_SYM opt_ident
           {
-	    Lex->alter_info.flags|= ALTER_DROP_INDEX;
+	    Lex->alter_info.flags|= ALTER_DROP_INDEX | ALTER_FOREIGN_KEY;
           }
 	| DROP PRIMARY_SYM KEY_SYM
 	  {