The pair took finger prick tests at a clinic in the capital, Bridgetown, to raise awareness about the fight against HIV/AIDS, joining other events held worldwide on Dec. 1.
"If you are positive and you catch it soon enough, you can take medication and have a completely normal life with your loved ones," Prince Harry said.
Rihanna, a native of the Caribbean island who now lives in New York, was surprised at the ease and speed of the test, which UNAIDS says is cheap, accurate and delivers results quickly.
"I dont think people are aware of how much progress has been made with the fight against HIV and AIDS," said the 28-year-old singer. "The more we get the word out, the better."
According to 2015 data from the Joint United Nations Programme on HIV/AIDS (UNAIDS), some 36.7 million people are living with HIV globally. Two million people are living with HIV in Latin America and the Caribbean.
Prince Harry and Rihanna were in Barbados separately to commemorate the 50th anniversary of the islands independence from British rule on Wednesday, Reuters reported.