David Foster was sketchy about his relationship with Katharine McPhee, but not anymore ... he wants you to know they're totally a couple!
David and Katharine were smooching it up during lunch Thursday in Bev Hills at E Baldi. They've been seen together a lot, and they were clearly together but never said so ... and now there's no need.
Gotta say ... David's got game. Linda Thompson, Yolanda Foster, etc. etc. He even dated Oksana Grigorieva.
BTW ... David is 68 and Katherine is 33.