diff --git a/mysql-test/suite/innodb_fts/r/alter_parser.result b/mysql-test/suite/innodb_fts/r/alter_parser.result
new file mode 100644
index 0000000000000..7cf8314731e99
--- /dev/null
+++ b/mysql-test/suite/innodb_fts/r/alter_parser.result
@@ -0,0 +1,50 @@
+#
+# MDEV-41758 ALTER TABLE ignores a changed fulltext parser of an index
+#
+INSTALL PLUGIN simple_parser SONAME 'mypluglib';
+# Adding a parser. The index is built with it: simple_parser keeps
+# 'abc-def' as one word, so 'abc' does not match.
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT, FULLTEXT ft(c)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1, 'abc-def');
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c) WITH PARSER simple_parser;
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `id` int(11) NOT NULL,
+  `c` text DEFAULT NULL,
+  PRIMARY KEY (`id`),
+  FULLTEXT KEY `ft` (`c`) WITH PARSER `simple_parser`
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_uca1400_ai_ci
+SELECT id FROM t WHERE MATCH(c) AGAINST('abc');
+id
+DROP TABLE t;
+# Removing it, which cannot be done instantly
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT,
+FULLTEXT ft(c) WITH PARSER simple_parser) ENGINE=InnoDB;
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c), ALGORITHM=INSTANT;
+ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c);
+SHOW CREATE TABLE t;
+Table	Create Table
+t	CREATE TABLE `t` (
+  `id` int(11) NOT NULL,
+  `c` text DEFAULT NULL,
+  PRIMARY KEY (`id`),
+  FULLTEXT KEY `ft` (`c`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_uca1400_ai_ci
+DROP TABLE t;
+# Adding it under another name: the index is rebuilt, not just renamed,
+# so 'abc' does not match.
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT, FULLTEXT ft(c)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1, 'abc-def');
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft2(c) WITH PARSER simple_parser;
+SELECT id FROM t WHERE MATCH(c) AGAINST('abc');
+id
+DROP TABLE t;
+# The same parser, in any letter case, is no change
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT,
+FULLTEXT ft(c) WITH PARSER simple_parser) ENGINE=InnoDB;
+ALTER TABLE t DROP INDEX ft,
+ADD FULLTEXT ft(c) WITH PARSER SIMPLE_PARSER, ALGORITHM=INSTANT;
+DROP TABLE t;
+UNINSTALL PLUGIN simple_parser;
diff --git a/mysql-test/suite/innodb_fts/t/alter_parser.test b/mysql-test/suite/innodb_fts/t/alter_parser.test
new file mode 100644
index 0000000000000..f5cc2b277b536
--- /dev/null
+++ b/mysql-test/suite/innodb_fts/t/alter_parser.test
@@ -0,0 +1,43 @@
+--source include/have_innodb.inc
+--source include/have_simple_parser.inc
+
+--echo #
+--echo # MDEV-41758 ALTER TABLE ignores a changed fulltext parser of an index
+--echo #
+
+INSTALL PLUGIN simple_parser SONAME 'mypluglib';
+
+--echo # Adding a parser. The index is built with it: simple_parser keeps
+--echo # 'abc-def' as one word, so 'abc' does not match.
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT, FULLTEXT ft(c)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1, 'abc-def');
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c) WITH PARSER simple_parser;
+SHOW CREATE TABLE t;
+SELECT id FROM t WHERE MATCH(c) AGAINST('abc');
+DROP TABLE t;
+
+--echo # Removing it, which cannot be done instantly
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT,
+  FULLTEXT ft(c) WITH PARSER simple_parser) ENGINE=InnoDB;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c), ALGORITHM=INSTANT;
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft(c);
+SHOW CREATE TABLE t;
+DROP TABLE t;
+
+--echo # Adding it under another name: the index is rebuilt, not just renamed,
+--echo # so 'abc' does not match.
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT, FULLTEXT ft(c)) ENGINE=InnoDB;
+INSERT INTO t VALUES (1, 'abc-def');
+ALTER TABLE t DROP INDEX ft, ADD FULLTEXT ft2(c) WITH PARSER simple_parser;
+SELECT id FROM t WHERE MATCH(c) AGAINST('abc');
+DROP TABLE t;
+
+--echo # The same parser, in any letter case, is no change
+CREATE TABLE t (id INT PRIMARY KEY, c TEXT,
+  FULLTEXT ft(c) WITH PARSER simple_parser) ENGINE=InnoDB;
+ALTER TABLE t DROP INDEX ft,
+  ADD FULLTEXT ft(c) WITH PARSER SIMPLE_PARSER, ALGORITHM=INSTANT;
+DROP TABLE t;
+
+UNINSTALL PLUGIN simple_parser;
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index cf987c052702f..fcff6b29f7b88 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -6972,6 +6972,13 @@ Compare_keys compare_keys_but_name(const KEY *table_key, const KEY *new_key,
       (new_key->flags & HA_KEYFLAG_MASK))
     return Compare_keys::NotEqual;
 
+  /* A fulltext parser added, removed or replaced */
+  if (((table_key->flags ^ new_key->flags) & HA_USES_PARSER) ||
+      ((table_key->flags & HA_USES_PARSER) &&
+       !Lex_ident_plugin(*plugin_name(table_key->parser)).streq(
+         *new_key->parser_name)))
+    return Compare_keys::NotEqual;
+
   if (table_key->user_defined_key_parts != new_key->user_defined_key_parts)
     return Compare_keys::NotEqual;