According to one embodiment, a memory system includes a nonvolatile memory and a controller. The controller selects, as a write mode, at least one of a first mode in which N-bit data is written per memory cell in the nonvolatile memory and a second mode in which M-bit data is written per memory cell in the nonvolatile memory as a write mode. N is equal to or larger than one. M is larger than N. The controller selects the second mode when a reception speed of data, which is received in accordance with acceptance of one or more write commands from the host, is equal to or slower than a threshold, and selects the first mode when the reception speed is faster than the threshold.