diff --git a/drivers/include/mtd_mapper.h b/drivers/include/mtd_mapper.h
index 9f2ddbc32d514..025ea95dc81b1 100644
--- a/drivers/include/mtd_mapper.h
+++ b/drivers/include/mtd_mapper.h
@@ -39,6 +39,7 @@
  *         .sector_count = SECTOR_COUNT / 2,
  *         .pages_per_sector = PAGE_PER_SECTOR,
  *         .page_size = PAGE_SIZE,
+ *         .write_size = WRITE_SIZE,
  *     },
  *     .parent = &parent,
  *     .sector = SECTOR_COUNT / 2
diff --git a/drivers/mtd_mapper/mtd_mapper.c b/drivers/mtd_mapper/mtd_mapper.c
index bca4bcc98ad1d..e02b35e0e5781 100644
--- a/drivers/mtd_mapper/mtd_mapper.c
+++ b/drivers/mtd_mapper/mtd_mapper.c
@@ -78,6 +78,7 @@ static int _init(mtd_dev_t *mtd)
     assert(backing_mtd->page_size == region->mtd.page_size);
     assert(backing_mtd->pages_per_sector == region->mtd.pages_per_sector);
     assert(backing_mtd->sector_count >= region->mtd.sector_count);
+    assert(backing_mtd->write_size == region->mtd.write_size);
 
     /* offset + region size must not exceed the backing device */
     assert(region->sector + region->mtd.sector_count <= backing_mtd->sector_count);
diff --git a/tests/mtd_mapper/main.c b/tests/mtd_mapper/main.c
index 5b073a6ab84c9..ef88a0f380a79 100644
--- a/tests/mtd_mapper/main.c
+++ b/tests/mtd_mapper/main.c
@@ -37,6 +37,9 @@
 #ifndef PAGE_SIZE
 #define PAGE_SIZE 64
 #endif
+#ifndef WRITE_SIZE
+#define WRITE_SIZE 4
+#endif
 
 #define MEMORY_SIZE         PAGE_SIZE * PAGE_PER_SECTOR * SECTOR_COUNT
 
@@ -176,6 +179,7 @@ static mtd_dev_t dev = {
     .sector_count = SECTOR_COUNT,
     .pages_per_sector = PAGE_PER_SECTOR,
     .page_size = PAGE_SIZE,
+    .write_size = WRITE_SIZE,
 };
 
 static mtd_mapper_parent_t _parent = MTD_PARENT_INIT(&dev);
@@ -186,6 +190,7 @@ static mtd_mapper_region_t _region_a = {
         .sector_count = SECTOR_COUNT / 2,
         .pages_per_sector = PAGE_PER_SECTOR,
         .page_size = PAGE_SIZE,
+        .write_size = WRITE_SIZE,
     },
     .parent = &_parent,
     .sector = 0,
@@ -197,6 +202,7 @@ static mtd_mapper_region_t _region_b = {
         .sector_count = SECTOR_COUNT / 2,
         .pages_per_sector = PAGE_PER_SECTOR,
         .page_size = PAGE_SIZE,
+        .write_size = WRITE_SIZE,
     },
     .parent = &_parent,
     .sector = SECTOR_COUNT / 2,