diff --git a/mysql-test/r/ctype_utf8.result b/mysql-test/r/ctype_utf8.result
index 364640210c9..d95b506f247 100644
--- a/mysql-test/r/ctype_utf8.result
+++ b/mysql-test/r/ctype_utf8.result
@@ -6182,6 +6182,22 @@ c0	c1	c2	c3	c4
 2012-06-11 15:18:24	2012-06-11 15:18:24	2012-06-11 15:18:24	2012-06-11 15:18:24	2012-06-11 15:18:24
 DROP TABLE t2, t1;
 #
+# MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+#
+SET NAMES utf8;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf8);
+INSERT INTO t1 VALUES (REPEAT('Ａ',100));
+SELECT OCTET_LENGTH(a) FROM t1;
+OCTET_LENGTH(a)
+300
+ALTER TABLE t1 MODIFY a TINYTEXT CHARACTER SET utf8;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SELECT OCTET_LENGTH(a),a FROM t1;
+OCTET_LENGTH(a)	a
+255	ＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡＡ
+DROP TABLE t1;
+#
 # End of 5.5 tests
 #
 #
diff --git a/mysql-test/r/ctype_utf8mb4.result b/mysql-test/r/ctype_utf8mb4.result
index abdec24bf37..ac53a7e5a4e 100644
--- a/mysql-test/r/ctype_utf8mb4.result
+++ b/mysql-test/r/ctype_utf8mb4.result
@@ -2809,6 +2809,22 @@ Warning	1292	Truncated incorrect INTEGER value: 'a'
 DROP TABLE t1;
 # End of test for Bug#13581962,Bug#14096619
 #
+# MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+#
+SET NAMES utf8mb4;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf8mb4);
+INSERT INTO t1 VALUES (REPEAT('😎',100));
+SELECT OCTET_LENGTH(a) FROM t1;
+OCTET_LENGTH(a)
+400
+ALTER TABLE t1 MODIFY a TINYTEXT CHARACTER SET utf8mb4;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SELECT OCTET_LENGTH(a),a FROM t1;
+OCTET_LENGTH(a)	a
+252	😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎😎
+DROP TABLE t1;
+#
 # End of 5.5 tests
 #
 #
diff --git a/mysql-test/r/myisam-blob.result b/mysql-test/r/myisam-blob.result
index 43db7c8badd..6b41a244621 100644
--- a/mysql-test/r/myisam-blob.result
+++ b/mysql-test/r/myisam-blob.result
@@ -29,9 +29,11 @@ select length(data) from t1;
 length(data)
 18874368
 alter table t1 modify data blob;
+Warnings:
+Warning	1265	Data truncated for column 'data' at row 1
 select length(data) from t1;
 length(data)
-0
+65535
 drop table t1;
 CREATE TABLE t1 (data BLOB) ENGINE=myisam;
 INSERT INTO t1 (data) VALUES (NULL);
diff --git a/mysql-test/r/type_blob.result b/mysql-test/r/type_blob.result
index 6a09df409da..132de86e27b 100644
--- a/mysql-test/r/type_blob.result
+++ b/mysql-test/r/type_blob.result
@@ -1002,9 +1002,42 @@ COUNT(*)
 DROP FUNCTION f1;
 DROP TABLE t1;
 End of 5.1 tests
+#
+# Start of 5.5 tests
+#
 CREATE TABLE t1 ( f1 blob, f2 blob );
 INSERT INTO t1 VALUES ('','');
 SELECT f1,f2,"found row" FROM t1 WHERE f1 = f2 ;
 f1	f2	found row
 		found row
 DROP TABLE t1;
+#
+# MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+#
+CREATE TABLE t1 (a MEDIUMBLOB);
+INSERT INTO t1 VALUES (REPEAT(0x61,128000));
+SELECT LENGTH(a) FROM t1;
+LENGTH(a)
+128000
+ALTER TABLE t1 MODIFY a BLOB;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SELECT LENGTH(a) FROM t1;
+LENGTH(a)
+65535
+DROP TABLE t1;
+CREATE TABLE t1 (a BLOB);
+INSERT INTO t1 VALUES (REPEAT(0x61,65000));
+SELECT LENGTH(a) FROM t1;
+LENGTH(a)
+65000
+ALTER TABLE t1 MODIFY a TINYBLOB;
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 1
+SELECT LENGTH(a) FROM t1;
+LENGTH(a)
+255
+DROP TABLE t1;
+#
+# End of 5.5 tests
+#
diff --git a/mysql-test/t/ctype_utf8.test b/mysql-test/t/ctype_utf8.test
index b791f78fc21..426985a6d42 100644
--- a/mysql-test/t/ctype_utf8.test
+++ b/mysql-test/t/ctype_utf8.test
@@ -1667,6 +1667,17 @@ SHOW CREATE TABLE t2;
 SELECT * FROM t2;
 DROP TABLE t2, t1;
 
+--echo #
+--echo # MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+--echo #
+SET NAMES utf8;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf8);
+INSERT INTO t1 VALUES (REPEAT('Ａ',100));
+SELECT OCTET_LENGTH(a) FROM t1;
+ALTER TABLE t1 MODIFY a TINYTEXT CHARACTER SET utf8;
+SELECT OCTET_LENGTH(a),a FROM t1;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 5.5 tests
 --echo #
diff --git a/mysql-test/t/ctype_utf8mb4.test b/mysql-test/t/ctype_utf8mb4.test
index 55909d6a6c8..3f2b6000597 100644
--- a/mysql-test/t/ctype_utf8mb4.test
+++ b/mysql-test/t/ctype_utf8mb4.test
@@ -1809,6 +1809,18 @@ DROP TABLE t1;
 
 --echo # End of test for Bug#13581962,Bug#14096619
 
+--echo #
+--echo # MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+--echo #
+SET NAMES utf8mb4;
+CREATE TABLE t1 (a TEXT CHARACTER SET utf8mb4);
+INSERT INTO t1 VALUES (REPEAT('😎',100));
+SELECT OCTET_LENGTH(a) FROM t1;
+ALTER TABLE t1 MODIFY a TINYTEXT CHARACTER SET utf8mb4;
+SELECT OCTET_LENGTH(a),a FROM t1;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 5.5 tests
 --echo #
diff --git a/mysql-test/t/type_blob.test b/mysql-test/t/type_blob.test
index 04b074b5cc5..a20173e231a 100644
--- a/mysql-test/t/type_blob.test
+++ b/mysql-test/t/type_blob.test
@@ -643,6 +643,10 @@ DROP TABLE t1;
 
 --echo End of 5.1 tests
 
+--echo #
+--echo # Start of 5.5 tests
+--echo #
+
 #
 # Problem when comparing blobs #778901
 #
@@ -651,3 +655,24 @@ CREATE TABLE t1 ( f1 blob, f2 blob );
 INSERT INTO t1 VALUES ('','');
 SELECT f1,f2,"found row" FROM t1 WHERE f1 = f2 ;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-9319 ALTER from a bigger to a smaller blob type truncates too much data
+--echo #
+CREATE TABLE t1 (a MEDIUMBLOB);
+INSERT INTO t1 VALUES (REPEAT(0x61,128000));
+SELECT LENGTH(a) FROM t1;
+ALTER TABLE t1 MODIFY a BLOB;
+SELECT LENGTH(a) FROM t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a BLOB);
+INSERT INTO t1 VALUES (REPEAT(0x61,65000));
+SELECT LENGTH(a) FROM t1;
+ALTER TABLE t1 MODIFY a TINYBLOB;
+SELECT LENGTH(a) FROM t1;
+DROP TABLE t1;
+
+--echo #
+--echo # End of 5.5 tests
+--echo #
diff --git a/sql/field.cc b/sql/field.cc
index 585abad2cdc..a7984c290de 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -7811,6 +7811,35 @@ uint32 Field_blob::get_length(const uchar *pos, uint packlength_arg)
 }
 
 
+/**
+  Copy a value from another BLOB field of the same character set.
+  This method is used by Copy_field, e.g. during ALTER TABLE.
+*/
+int Field_blob::copy_value(Field_blob *from)
+{
+  DBUG_ASSERT(field_charset == from->charset());
+  int rc= 0;
+  uint32 length= from->get_length();
+  uchar *data;
+  from->get_ptr(&data);
+  if (packlength < from->packlength)
+  {
+    int well_formed_errors;
+    set_if_smaller(length, Field_blob::max_data_length());
+    length= field_charset->cset->well_formed_len(field_charset,
+                                                 (const char *) data,
+                                                 (const char *) data + length,
+                                                 length, &well_formed_errors);
+    rc= report_if_important_data((const char *) data + length,
+                                 (const char *) data + from->get_length(),
+                                 true);
+  }
+  store_length(length);
+  bmove(ptr + packlength, (uchar*) &data, sizeof(char*));
+  return rc;
+}
+
+
 int Field_blob::store(const char *from,uint length,CHARSET_INFO *cs)
 {
   ASSERT_COLUMN_MARKED_FOR_WRITE_OR_COMPUTED;
diff --git a/sql/field.h b/sql/field.h
index 7199e40c173..81fae343cc1 100644
--- a/sql/field.h
+++ b/sql/field.h
@@ -2959,6 +2959,11 @@ protected:
   */
   String value;
   
+  void store_length(uchar *i_ptr, uint i_packlength, uint32 i_number);
+  inline void store_length(uint32 number)
+  {
+    store_length(ptr, packlength, number);
+  }
 public:
   Field_blob(uchar *ptr_arg, uchar *null_ptr_arg, uchar null_bit_arg,
 	     enum utype unireg_check_arg, const char *field_name_arg,
@@ -3036,11 +3041,6 @@ public:
   int reset(void) { bzero(ptr, packlength+sizeof(uchar*)); return 0; }
   void reset_fields() { bzero((uchar*) &value,sizeof(value)); }
   uint32 get_field_buffer_size(void) { return value.alloced_length(); }
-  void store_length(uchar *i_ptr, uint i_packlength, uint32 i_number);
-  inline void store_length(uint32 number)
-  {
-    store_length(ptr, packlength, number);
-  }
   inline uint32 get_length(uint row_offset= 0)
   { return get_length(ptr+row_offset, this->packlength); }
   uint32 get_length(const uchar *ptr, uint packlength);
@@ -3069,6 +3069,7 @@ public:
   {
     set_ptr_offset(0, length, data);
   }
+  int copy_value(Field_blob *from);
   uint get_key_image(uchar *buff,uint length, imagetype type);
   void set_key_image(const uchar *buff,uint length);
   Field *new_key_field(MEM_ROOT *root, TABLE *new_table,
diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index 0f6c85f50e8..509c31dfa86 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -337,9 +337,7 @@ static void do_copy_next_number(Copy_field *copy)
 
 static void do_copy_blob(Copy_field *copy)
 {
-  ulong length=((Field_blob*) copy->from_field)->get_length();
-  ((Field_blob*) copy->to_field)->store_length(length);
-  memcpy(copy->to_ptr, copy->from_ptr, sizeof(char*));
+  ((Field_blob*) copy->to_field)->copy_value(((Field_blob*) copy->from_field));
 }
 
 static void do_conv_blob(Copy_field *copy)
@@ -714,12 +712,7 @@ Copy_field::get_copy_func(Field *to,Field *from)
     if (!(from->flags & BLOB_FLAG) || from->charset() != to->charset())
       return do_conv_blob;
     if (from_length != to_length)
-    {
-      // Correct pointer to point at char pointer
-      to_ptr+=   to_length - portable_sizeof_char_ptr;
-      from_ptr+= from_length - portable_sizeof_char_ptr;
       return do_copy_blob;
-    }
   }
   else
   {