Kim Kardashian and Kanye West in Rare PDA Moment

About the only thing Kim Kardashian and Kanye West don't do on social media is PDA -- but, uhh ... go ahead and scratch that off the list.

Kimye was hanging Thursday night at Kris Jenner's house .. where the fam celebrated Kourtney's 39th birthday. Kim floated across the room -- in a skin tight white outfit -- and plopped herself down in Kanye's lap. And then, boom! They started making out.

Cute moment for them ... super awkward one for a friend sitting alone on the couch behind them, who mumbled, "You guys are the best married couple I know."

Great color commentary!

You can tell Kanye was in a great mood. The bday bash turned into a listening party for his 2 upcoming albums. It's getting killer reviews ... at least from his wife.