The singer's celebrating her milestone bday -- which was Tuesday -- by traveling to Antigua with her crew for what she's dubbed #shanbday week ... so you know the celebrating has just begun.

Based on the photos from photographer Phillip Silverstein, it looks pretty damn good so far. Ashanti's flaunting her bikini bod in a sexy silver swimsuit while snacking on watermelon. Undoubtedly juicy ... and refreshing.

Ashanti's also spending time riding jet skis and going to fancy dinners while rocking some fine formal wear. In other words ... she's doing it right.

She thanked all of her fans Wednesday for all the bday love she's been getting ... adding, "I cannot express how happy I am for this next chapter in life."