Jeff Goldblum -- Bootyful Beach Day with Much Younger GF

Jeff Goldblum is living the dream and holding on to the dog days of summer ... and to his much younger girlfriend's ass.

While vacationing in St. Barts on Wednesday, the 59-year-old shirtless stud enjoyed a very hands-on day in the sun with his bikini-clad 29-year-old piece Emilie Livingston.

When you're a mac daddy like Goldblum, life really is a day at the beach.