The singer's 21-year-old cousin was shot several times Tuesday night when he was approached by a man.
In an Instagram post, Rihanna said they had just spent Christmas Day together.
"RIP cousin... can't believe it was just last night that I held you in my arms! never thought that would be the last time I felt the warmth in your body!!! Love you always man!" her post read, ending with the hashtag #endgunviolence.
Authorities were searching the island for the shooter.