Rihanna — 25th Birthday with Chris Brown … and TINY BIKINI

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Rihanna turned 25 years old yesterday … and she celebrated by throwing on one of her smallest bikinis and hitting a jet ski in Hawaii … with Chris Brown.

RiRi and Brown spent the whole day together … holding hands, jetting around in the ocean … and probably smoking weed.

There were rumors that Brown was planning to pop the question during their Hawaii vacay … but there doesn”t appear to be a ring on her finger … yet.