Say'un is a city in Hadhramaut region, Hadhramaut Governorate of Yemen. It is located in the middle of the Hadhramaut valley, about 360 km from Mukalla, the capital of Mukalla District and the largest city in the area, via western route. It is also 12 km away from Shibam and 35 km away from Tarim, other large cities in Hadramaut valley.

It is said that the origin of the city was a resting point for travelers. There was a cafe with a woman named Seiyun there, and the area was named after her in her honor. Since then, the city has become the largest area of Hadhramaut valley. The most prominent villages surrounding the city including Madurah, Merida, Burr, Hazkir, and Houta Sultana.

It is served by Seiyun Airport. It is also well known for the Seiyun Palace of the Sultan of Kathiri built in 1920s by mud and stones. It is overseeing the market place for merchants coming from outside.