Posen called the role "an amazing opportunity" and said he hoped to give a new perspective to the competing designers.
Kors isn't "out" - as host Heidi Klum likes to say to contestants who get kicked off the fashion reality TV show. His schedule didn't allow a commitment, but he will come back as a guest judge for the season finale, according to a statement for Lifetime TV.
Another twist to the revamped Project Runway - which airs in the UK on Sky1 HD - is that contestants compete as teams instead of individuals.
The Project Runway season starts on January 24.