The actor is celebrating his 66th birthday today and he's marking the occasion with a topless thirst trap ... showing off his abs, which are still there.

Kevin's posing outdoors somewhere, leaning back on what appears to be a rock near the base of a bluff ... and it looks like he has swim trunks on, so he may be near a beach.

Seems Kevin even had someone block out the sun here to get the photo lighting just right ... he may have a future as an Instagram influencer.

KB captioned the thirst trap ... "This is 66" with a birthday cake emoji.

The post is working, if it's indeed an attention grab ... he's getting tons of birthday love and the replies are mostly filled with flattering comments.

Frankly, Kevin's looking super fit these days ... especially for a 66-year-old.