Emperor Go-Sai (後西天皇 Go-Sai-tennō), also known as Emperor Go-Saiin (後西院天皇 Go-Saiin-tennō) was the 111th emperor of Japan, according to the traditional order of succession.
Go-Sai's reign spanned the years from 1655 through 1663.
This 17th-century sovereign was named after the 9th-century Emperor Junna and go- (後), translates literally as later, and thus, he could have been called the "Later Emperor Junna". Emperor Go-Sai could not pass the throne onto his descendants. For this reason, he was known as the Go-Saiin emperor, after an alternate name of Emperor Junna, who had confronted and reached an accommodation with similar issues. This emperor was also called "Emperor of the Western Palace" (西院の帝 Saiin no mikado). The Japanese word go has also been translated to mean the second one, and thus, this emperor might be identified as "Junna II". During the Meiji Era, the name became just Go-Sai.