"Caddyshack" star Michael O'Keefe is back on the bag ... the actor who famously played Danny Noonan in the iconic 1980 golf flick is a real-life caddie at the U.S. Open this week!!!

For real!!!!!

Here's the deal ... O'Keefe used to work at Winged Foot Golf Course in N.Y. back in the 1970s before his acting career took off -- and with the PGA Tour's U.S. Open scheduled to play there this year, Michael had an idea.

O'Keefe told the New York Post he wanted to reprise his role as Noonan and hop on a pro's bag ... all in an effort to help raise money for caddies who have been affected by the pandemic.

Of course, no pro was going to let him be their caddie during ACTUAL tournament play ... but O'Keefe at least found a taker for some practice rounds!!

Danny Balin -- a head pro at a local Long Island country club -- offered to let O'Keefe caddy for him Monday and Tuesday ... and the early pics from their outing are priceless.

Check out how Noonan looks today ... not a thing's changed!!!

Balin says he re-watched "Caddyshack" in anticipation of his rounds with O'Keefe ... and promised he'd have all the 1-liners down by the time the two hit the fairways.

Well, we're waiting!!!!