Soyuz TMA-21 ("Gagarin") was a Soyuz flight to the International Space Station (ISS). It transported three members of the Expedition 27 crew to the ISS, and docked at the station on April 6, 2011. TMA-21 is the 109th flight of a Soyuz spacecraft, the first of which launched in 1967. The Soyuz remained attached to the space station as a lifeboat, throughout the remainder of Expedition 27 and through the end of Expedition 28, and returned to Earth on September 16, 2011.

Expedition 27 was the 27th long-duration expedition to the International Space Station (ISS), starting on 16 March 2011. Expedition 27 saw numerous notable events, including the undocking of the Progress M-09M and Kounotori 2 spacecraft, the arrival of the Soyuz TMA-21 and Progress M-10M spacecraft, and the final rendezvous with the ISS of NASA's Space Shuttle Endeavour, on its last mission, STS-134. The expedition ended on 23 May 2011 with the departure of the Soyuz TMA-20 spacecraft, although command of the station was ceremonially handed over to the crew of Expedition 28 on 22 May.