Provided are memory devices configured to perform row hammer handling operations, and memory systems including such memory devices. An example memory device may include a memory cell array including a plurality of memory cell rows; a row hammer handler that is configured to determine whether to perform a row hammer handling operation to refresh adjacent memory cell rows adjacent to a first row that is being intensively accessed from among the memory cell rows, resulting in a determination result; and a refresh manager configured to perform either a normal refresh operation for sequentially refreshing the memory cell rows or the row hammer handling operation, based on the determination result of the row hammer handler.