The Stand by Me actor died in 1993 from a drug overdose at the age of 23 and his sibling Rain Phoenix, 51, has now shared a string of photos of him on her Instagram in his memory.
Her photo carousel included a number of quotes attributed to River, and she simply captioned it: "Happy Birthday River."
Red Hot Chili Peppers bassist Flea, 61, who was friends with River and there when the actor collapsed outside Los Angeles nightclub The Viper Room before his death and accompanied River to the hospital, said in a comment on Rain's post: "Happy birthday most beautiful one."
Late actor River Phoenix's breakout role was in the 1986 coming of age film Stand By Me. (AP PHOTO)
River was the oldest of the five Phoenix siblings, who along with Rain included Oscar-winning Joker actor Joaquin, 49, as well as Liberty, 48 and 45-year-old Summer.
They grew up travelling with their parents as missionaries for the Children of God.
After separating from the religious movement, the family settled in California in the late 1970s, where River started his acting career.Â
His feature film debut was 1985's Explorers, with his breakout role in Stand by Me a year later.
The coming-of-age story told of a gang of high school children on the hunt for a corpse, and was adapted from the Stephen King story The Body.
River was then Oscar-nominated in 1988 for his role in Running on Empty and appeared as a young Indiana Jones in 1989's Indiana Jones and the Last Crusade.
Rain and Joaquin were present at the scene of River's death on October 31, 1993 with the actor's brother calling 911 to the scene and Rain attempting mouth-to-mouth resuscitation.
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