The "Tonight Show" host, 44, 'fessed up to having a crush on his "Almost Famous" co-star, 39, and being passed up for Hudson's now ex-husband, Black Crowes singer Chris Robinson.
The big reveal happened Monday night while Fallon and his guest, actress Margot Robbie, played a game of "Loaded Questions," which forced them to share potentially embarrassing secrets.
Toward the end of the game, both were forced to simultaneously answer the question, "Do you have a co-star you thought you could have dated but didn't?"
While Robbie revealed she'd never had an on-set crush, Fallon blurted out Hudson's name.
"We were, like, super good friends," he recalled about his time on set with Hudson.
One night while shooting the 2000 drama, Fallon took his co-star to see Canadian comedy troupe The Kids in the Hall, where the pair met singer Robinson for the first time.
Fallon, then a cast member on "Saturday Night Live," told Robinson he was a "big fan" of his band.
The following day, Fallon and Hudson had plans to ice skate at Central Park, but beforehand Hudson asked to borrow the late-night funnyman's Black Crowes CDs.
At the skating rink, oblivious Fallon finally worked up the nerve to ask Hudson out.
"We ice skated maybe three or four times around the rink," Fallon remembered. "And then I go, ‘So, what are you doing now?' She goes, ‘I've got to go because I have a date with Chris Robinson of the Black Crowes.'"
Hudson and Robinson hit it off and tied the knot on Dec. 31, 2000. The pair welcomed a son, Ryder, 14, before divorcing in 2007. Hudson later dated and had a son, Bingham, 7, with ex Matthew Bellamy, and recently welcomed daughter Rani Rose with boyfriend Danny Fujikawa.
As for Fallon, he eventually found love, too — thanks to a different co-star.
Watch Fallon tell his hilarious story in the video above!