Rihanna's cousin was shot and killed this week ... just a day after she'd seen him for Christmas in her native home of Barbados.
RiRi posted a tribute Tuesday to her late relative, Tavon Alleyne, saying, "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! #endgunviolence."
Tavon -- who was just 21 years old -- was shot several times by an unidentified man as he was walking near his home in the St. Michael area of the island.
Cops are on the lookout for the shooter, and have asked the public for any info or leads that could lead to his capture.
RIP