A memory system includes a nonvolatile memory that includes a plurality of blocks and a memory controller with a control circuit. The control circuit executes a first garbage collection for a first stream, which includes reading valid first data stored in a first block associated with the first stream and valid second data stored in a second block associated with the first stream and writing the read first data and the read second data into a third block associated with the first stream, and a second garbage collection for a second stream, which includes reading valid third data stored in a fourth block associated with the second stream and valid fourth data stored in a fifth block associated with the second stream and writing the read third data and the read fourth data into a sixth block associated with the second stream.