diff --git a/mysql-test/main/long_unique_innodb.result b/mysql-test/main/long_unique_innodb.result
index fd2b3daf8e5..8c23d61f2bc 100644
--- a/mysql-test/main/long_unique_innodb.result
+++ b/mysql-test/main/long_unique_innodb.result
@@ -169,4 +169,15 @@ id	f
 1	foo
 2	foo
 drop table t1;
+#
+# MDEV-39846 UNIQUE BLOB contains duplicate records after ALTER IGNORE with READ COMMITTED
+#
+create table t1 (pk int primary key, b blob) engine=innodb;
+insert into t1 values (1,'foo'),(2,'foo');
+set transaction isolation level read committed;
+alter ignore table t1 add unique (b);
+select * from t1;
+pk	b
+1	foo
+drop table t1;
 # End of 10.6 tests
diff --git a/mysql-test/main/long_unique_innodb.test b/mysql-test/main/long_unique_innodb.test
index cbe2d7431fe..fc51817e45a 100644
--- a/mysql-test/main/long_unique_innodb.test
+++ b/mysql-test/main/long_unique_innodb.test
@@ -176,4 +176,14 @@ update ignore t1 set id = 2;
 select * from t1;
 drop table t1;
 
+--echo #
+--echo # MDEV-39846 UNIQUE BLOB contains duplicate records after ALTER IGNORE with READ COMMITTED
+--echo #
+create table t1 (pk int primary key, b blob) engine=innodb;
+insert into t1 values (1,'foo'),(2,'foo');
+set transaction isolation level read committed;
+alter ignore table t1 add unique (b);
+select * from t1;
+drop table t1;
+
 --echo # End of 10.6 tests
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index bb4993bec96..533c9d720ff 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -94,6 +94,7 @@ class Enable_wsrep_ctas_guard
 #endif /* WITH_WSREP */
 
 #include "sql_debug.h"
+#include "scope.h"
 
 #ifdef _WIN32
 #include <io.h>
@@ -10647,6 +10648,12 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
   */
   table_list->required_type= TABLE_TYPE_NORMAL;
 
+  enum_tx_isolation iso_level_initial= thd->tx_isolation;
+  SCOPE_EXIT([thd, iso_level_initial](){
+    thd->tx_isolation= iso_level_initial;
+  });
+  thd->tx_isolation= ISO_REPEATABLE_READ;
+
   DEBUG_SYNC(thd, "alter_table_before_open_tables");
 
   thd->open_options|= HA_OPEN_FOR_ALTER;