The 'Stay' singer is seen sitting on a countertop with her arm around Brown and her leg draped over his in one of the cosiest pictures since their reunion was confirmed last month.
The image was accompanied by the caption: "#phuckyoballoons ha! My birthday owned hoes."
Earlier Rihanna Instagramed a picture of herself in a revealing bikini top sitting on Brown's lap, apparently taken while they travelled through Hawaii.
The Bajan star also displayed Polaroids of herself looking loved up with Brown, one of which was captioned: "Like #diamonds in the sky," a lyric from her number one single 'Diamonds'.
In photos published in UK tabloids, the couple were shown holding hands and cuddling up on a beach, with Rihanna wearing a thong and high-waisted fishnet tights.
Rihanna previously marked her birthday by posing in a bikini while wearing her apparent engagement ring, as well as sharing with fans a snap from her 20th party in 2008, her last birthday before Brown assaulted her in February the following year.