The Vienna, Va.-based company said Wednesday it has signed an agreement with the Federal Space Agency of the Russian Federation and Rocket Space Corporation Energia for the additional seats.
The seats are being made available because of the increased production of the Soyuz spacecraft.
Space Adventures says the flights will be about 10 days.
Space Adventures first launched a private client into space in 2001. It has since arranged seven additional missions to the International Space Station. Its advisory board includes Apollo 11 moonwalker Buzz Aldrin and other astronauts.