According to one embodiment, a memory system includes a non-volatile memory and a controller. The controller manages validity of data in the non-volatile memory using a data map. The data map includes first fragment tables. Each of the first fragment tables stores first and second information. The first information indicates the validity of each data having a predetermined size written in a range of physical address in the non-volatile memory allocated to the first fragment table. The second information indicates the validity of a plurality of data having a predetermined size in each of entries. The controller selects a write destination block based on a size of write data to be written to the non-volatile memory by a write command from a host.