The 75-year-old veteran musician, who was close friends with the princess, shared a photo on Instagram of them laughing together to commemorate the occasion.
Diana, the mother of the Dukes of Cambridge and Sussex, was 36 when she was killed in a car crash in Paris on August 31 1997.
In 1981, when Prince Andrew turned 21, Sir Elton was reportedly hired to perform at the birthday celebration and it was there that he met Diana and the pair bonded over a Charleston dance on the dancefloor.
They became close friends, sharing a passion for raising awareness of Aids.
Following her death in 1997, Sir Elton performed at Diana’s funeral after the lyrics to his hit song Candle In The Wind were adapted to reflect her life.
The song tapped into the nation’s grief and went on to become the biggest-selling single in UK chart history up to that date.
The Rocketman singer has regularly supported her sons William and Harry over the years and attended both their royal weddings alongside his husband, David Furnish.