Turkmenistan participated in the referendum held in March 1991 in an attempt to preserve the Soviet Union as a renewed federation with the 98.26% voters approved.

Following the events of the failed coup that took place in August, the Supreme Soviet of the Turkmenistan decided to adopt the law "About Independence and Bases of a State System of Turkmenistan" when it declared independence on October 27, 1991. After disintegration of the USSR the Turkmen Soviet Socialist Republic became one of the last to the Soviet Socialist Republic on the former Soviet Union, proclaimed the state sovereignty.

According to wikipedia