The Avengers actor, 48, joined the singer on stage at Sting's 60th birthday bash at the Beacon Theatre in NYC back in October 2011.
In the two-year-old footage, Downey Jr. flawlessly belts out Police's 1980 hit "Driven to Tears" on stage, while the birthday boy accompanies the actor with backup vocals and guitar. Guests who joined Sting for his big 6-0 included Bruce Springsteen, Lady Gaga, Billy Joel, Stevie Wonder, and Herbie Hancock.
At the time the video was taken, Downey was busy promoting the second installment of Sherlock Holmes, A Game of Shadows. It's also worth noting that it was not the first time Sting and the actor shared a stage together. In Ally McBeal's fourth season, which aired from 2000 to 2001, Sting and Downey (who starred on the Fox show during the same season) performed a duet of "Every Breath You Take."
In fact, Downey was credited with bringing Sting as a special guest on the show, according to ABC News: "I think that it was through Robert and Sting's friendship that it happened," said Ally McBeal regular Vonda Shepard in an interview. "I know that they're good friends."
While he helped fete Sting's big 60th, the actor most recently threw a star-studded, weekend-long party celebrating wife Susan Downey's 40th birthday back in November.
Watch the impressive duet now!