diff --git a/mysql-test/main/vector2.result b/mysql-test/main/vector2.result
index 1d3ccbea13d..1273a214173 100644
--- a/mysql-test/main/vector2.result
+++ b/mysql-test/main/vector2.result
@@ -436,3 +436,21 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t	ALL	NULL	NULL	NULL	NULL	2	Using filesort
 drop table t;
 # End of 11.7 tests
+#
+# MDEV-35309 - ALTER performs vector truncation without WARN_DATA_TRUNCATED or similar warnings/errors
+#
+create table t (v vector(2));
+insert into t values (0x3131313132323232);
+select * from t;
+v
+11112222
+alter table t modify v vector(1);
+ERROR 01000: Data truncated for column 'v' at row 1
+set statement sql_mode='' for alter table t modify v vector(1);
+Warnings:
+Warning	1265	Data truncated for column 'v' at row 1
+select * from t;
+v
+1111
+drop table t;
+# End of 11.8 tests
diff --git a/mysql-test/main/vector2.test b/mysql-test/main/vector2.test
index ca50dd5427e..087d9a8916e 100644
--- a/mysql-test/main/vector2.test
+++ b/mysql-test/main/vector2.test
@@ -318,3 +318,17 @@ insert into t values (0x00000000),(0x00000000);
 explain select vec_totext(a) from t order by vec_distance_euclidean(a,0x00000000) limit 1;
 drop table t;
 --echo # End of 11.7 tests
+
+--echo #
+--echo # MDEV-35309 - ALTER performs vector truncation without WARN_DATA_TRUNCATED or similar warnings/errors
+--echo #
+create table t (v vector(2));
+insert into t values (0x3131313132323232);
+select * from t;
+--error WARN_DATA_TRUNCATED
+alter table t modify v vector(1);
+set statement sql_mode='' for alter table t modify v vector(1);
+select * from t;
+drop table t;
+
+--echo # End of 11.8 tests
diff --git a/sql/sql_type_vector.cc b/sql/sql_type_vector.cc
index b8d7f385c5d..8cb87ccbf9e 100644
--- a/sql/sql_type_vector.cc
+++ b/sql/sql_type_vector.cc
@@ -256,7 +256,22 @@ static void do_copy_vec(const Copy_field *copy)
     int2store(copy->to_ptr, to_length);
 
   if (from_length > to_length)
+  {
     memcpy(to, from, to_length);
+    if (copy->from_field->table->in_use->count_cuted_fields >
+        CHECK_FIELD_EXPRESSION)
+    {
+      for (uint i= to_length; i < from_length; i++)
+      {
+        if (from[i] != 0)
+        {
+            copy->to_field->set_warning(Sql_condition::WARN_LEVEL_WARN,
+                                        WARN_DATA_TRUNCATED, 1);
+            break;
+        }
+      }
+    }
+  }
   else
   {
     memcpy(to, from, from_length);