Sofia Vergara -- Looks Like an ENGAGEMENT RING!

"Modern Family" hot mama Sofia Vergara celebrated her big 40th birthday on Tuesday by rocking a sexy bikini in Mexico ... and flashing what is reportedly her new giant engagement ring.

Sofia's on-again-off-again businessman boyfriend Nick Loeb reportedly popped the question this week and gave her the huge bling during their vacation with pals on the Riviera Maya.

Sofia -- who dated Tom Cruise back in 2005, before the whole Katie Holmes thing -- has a son from a previous marriage.