Theyskens designed a capsule collection for Theory presented during the Spring/Summer 2011 Mercedes-Benz Fashion Week.
He will design the full Theory collection starting with Fall 2011.
Theyskens, 33, has been a darling of the fashion world since being tapped as Rochas creative director in 2002.
The designer was loved by critics but gained a reputation for creating luxurious garments too expensive for a commercial fashion line.
In 2006, he received the Council of Fashion Designers of America's International Award. He joined Nina Ricci in November of that year but his contract was not renewed in 2009.
Theory, known for its chic basics, was founded in New York City in 1997 and is now owned by a Japanese holding company.
Meanwhile, read the speculation that Theory is buying stake in Proenza Schouler.