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.

The launch of Soyuz TMA-21 was devoted to the 50th anniversary of the first crewed space mission, which was conducted by Yuri Gagarin on April 12, 1961.

The Soyuz is a Soviet crewed spacecraft, developed to perform lunar missions with crew. This version called 7K will fly 4 times on the giant launcher N1, and several tens of times on Proton to fly over the Moon, which will be successful during the mission Zond 4. Soyuz will become the first spacecraft to transport living beings on the Moon during the flight of Zond 5, with two turtles. Thereafter, it is adapted to the low orbit and will fly on the Soyuz launcher to supply the Soviet Salyut and Mir stations, and the ISS.