Britney Spears is Spring Breakin' in Hawaii and the 35-year-old mom of 2 is as ripped as any college chick out there.

On the heels of announcing her "Piece of Me" residency in Vegas will end in December ... Brit hit the beach on Kauai in a white kinda see-through bikini, and literally frolicked in the surf and sand.

She appeared to be doing some ab exercises while she was out there. It's clearly paying off.

As they say ... always leave 'em wanting more. This oughta do it.