A refresh controller of a memory device may include a timing controller, a refresh counter and an address generator. The timing controller generates a counter refresh signal in response to receiving a refresh command provided from an external device, and generates a hammer refresh signal that is activated periodically. The refresh counter generates a counter refresh address signal in response to the counter refresh signal, such that the counter refresh address signal represents a row address, the refresh counter being configured sequentially change the counter refresh address signal. The address generator generates a hammer refresh address signal in response to the hammer refresh signal, the hammer refresh address signal representing an address of a row of the memory device that is physically adjacent to a row of the memory device corresponding to a hammer address that is accessed intensively.