Britney Spears' Engagement Ring -- FEAST YOUR EYES

Voila -- the first photos of Britney Spears' new engagement ring ... snapped last night as the singer and fiancé Jason Trawick partied the night away in Las Vegas.

3-carat round stone ... diamond-paved band ... not too shabby for the world's favorite 30-year-old pop legend.

TMZ broke the story -- Britney's hosted the intimate engagement celebration at the Planet Hollywood Resort and Casino, before taking the ring public at Jason Trawick's big celebratory dinner at the Chocolate Lounge in the Sugar Factory Resturant.

After the big dinner ... they hit up the Chateau nightclub and danced until the world ends.