Sergei Vasilyevich Avdeyev (Сергей Васильевич Авдеев; born 1 January 1956) is a former Russian engineer and cosmonaut.

He was selected as a cosmonaut as part of the Energia Engineer Group 9 on 26 March 1987.

Avdeyev at one point held the record for cumulative time spent in space with 747.59 days in Earth orbit, accumulated through three tours of duty aboard the Mir Space Station.

[1][2][3] In his 747 days aboard Mir, cumulative across three missions, he went approximately 27,360 km/h and thus aged roughly 0.02 seconds (20 milliseconds) slower from an Earthbound person's perspective,[4] which is considerably more than any other human being, except Sergei Krikalev.

A common misconception is that the Apollo program astronauts hold the record—they did go faster than Avdeyev, but they were only in space for a few days.