ACK, Ack or Acks may refer to:
Computing Acknowledgement (data networks) ACK (TCP), the control character used in the Transmission Control Protocol to acknowledge receipt of a packet
Amsterdam Compiler Kit, a retargetable compiler suite and toolchain
People Ack Kinmonth, 21st century Australian composer Ack van Rooyen (1930–2021), Dutch jazz trumpeter and flugelhornist Ron Acks (1944–2023), American National Football League player
Transportation Nantucket Airlines or ACK Air, an American airline Nantucket Memorial Airport, Massachusetts, US (IATA airport code ACK) Acklington railway station, Acklington, Northumberland, UK (station code "ACK")
Other uses Anglican Church of Kenya Amar Chitra Katha, an Indian comic book publisher Assumption College, Kilmore, an Australian school Akakhora, an extinct Great Andamanese language (ISO 639-3 code "ack") TNK2 or ACK or ACK1, an enzyme that in humans is encoded by the TNK2 gene
See also Armstrong Whitworth F.K.8, an early biplane known as the "Big Ack" Ack Ack (disambiguation)