Megan Fox -- Engaged Again!

Brian Austin Green put a ring on it ... for the second time.

According to Us Magazine, BAG popped the question to girlfriend Megan Fox while vacationing in Hawaii June 1 -- but they lost the ring after she said "yes!"

The mag says the 2-carat ring was not recovered, even after the staff at the Four Seasons looked for it for hours.

The couple was previously engaged, but called it off last year.

Hopefully this time it sticks!