Say'un (Arabic: سيئون‎, also transliterated as Saywun or Seiyun, MSA pronunciation [seɪˈʔuːn], Hadhrami Arabic [seːˈwuːn]) is a town in the Hadhramaut region of Yemen. Postage stamps from the former Aden Protectorate sultanate of Kathiri (Aden-Kathiri) are sometimes inscribed "Kathiri State of Seiyun." As of 2006 the population was 75,700.

The most prominent landmark is the Sultan Al Kathiri Palace, built in 1920s.

It is served by Seiyun Airport.