The classical recording artist, once married to Lord Andrew Lloyd Webber, will be part of a three-person crew flying to the ISS.
After completing a tour in 2013, Sarah Brightman will embark on six months of preparation at the Star City cosmonaut training centre in Moscow.
She will be the eighth space tourist to visit the ISS.
Once there, she says she intends to become the first professional musician to sing from space.
“This voyage is a product of a dream, my dream. Finally it can be a reality. I am more excited about this than anything I have done in my life to date,” she told a news conference in Moscow.
She added that the schedule for her flight would “be determined very shortly by (Russian space agency) Roscosmos and the ISS partners.”
Space Adventures, the Vienna-based company that organizes flights for private spacefarers, did not disclose how much Sarah Brightman had paid for her seat on the Soyuz.
But the last space tourist, Cirque du Soleil chief executive Guy Laliberte, paid $35 million for the privilege.
Sarah Brightman began her career with the dance troupe Hot Gossip, which had a chart hit in 1978 with I Lost My Heart to a Starship Trooper.
She subsequently starred on the West End stage in Cats and Phantom of the Opera, both penned by Andrew Lloyd Webber, whom she married in 1984.
The pair divorced in 1990 and Sarah Brightman embarked on a solo singing career. She helped popularize the classical crossover genre, scoring a worldwide hit with her duet with Italian tenor Andrea Bocelli, Time To Say Goodbye.
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