The 28-year-old, who has won 42 appearances for his country, has insisted that he cannot wait for the "new challenge".
"It is a new challenge in my life. When I signed, the weather was very nice outside - it's a sign," he told reporters at a press conference.
"I cannot wait to arrive and to settle in Paris and to play in the Top 14. I am joining a great team and it would be brilliant to win the title next season, something they haven't achieved since 2007."
Steyn had previously been linked with a host of other Top 14 sides.