The photograph has been captioned, "Like #diamonds in the sky" - lyrics from Rihanna's latest hit - but it's her smile that says it all.
The star shared a number of other snaps taken at her party, which appears to have been held in a private hotel suite, including her birthday balloons and one of her eating her cake.
Earlier in the day, Rihanna shared a romantic walk hand-in-hand along the beach with Chris at she celebrated turning 25.
The on-off couple were seen strolling along enjoying the Hawaiian sunshine before sitting down to take in the view.
Rihanna and Chris, 23, went public with their relationship at the Grammys earlier this month. Their reconciliation was one of the big talking points of the night following Chris' conviction for assaulting the chart-topper in 2009.
The We Found Love singer hasn't been afraid to discuss her decision to reunite with the rapper, however, and she recently spoke frankly about the controversy surrounding it.
"Even if it's a mistake, it's my mistake," she said. "I'd rather just live my truth and take the backlash. I can handle it."
Rihanna is relaxing in Hawaii after debuting her range for River Island at London Fashion Week.
One star wishing she could have been on the island to celebrate that success and also join in with the birthday festivities is Katy Perry, who gave a shout out to her best friend on Twitter.