酒井氏
was a feudal domain in Edo period Japan, located in Dewa Province (modern-day Yamagata Prefecture), Japan. It was centered on Tsuruoka Castle in what is now the city of Tsuruoka in Yamagata Prefecture, and was thus also known as the . It was governed for the whole of its history by the Sakai clan, which resulted in an unusually stable and prosperous domain. During their rule over Shōnai, the Sakai clan was ranked as a family, and as such, had the privilege of shogunal audiences in the Great Hall ("Ohiroma") of Edo Castle. In the Boshin War of 1868–69, the domain joined the "Ōuetsu Reppan Dōmei", the alliance of northern domains supporting the Tokugawa shogunate, but then later defected to the imperial side. As with all other domains, it was disbanded in 1871.