The 43-year-old Tonight Show host recounted his early friendship with Timberlake and Spears while playing Plead the Fifth on Thursday's episode of Watch What Happens Live.
Fallon, who remains pals with Timberlake, confirmed the singer explained his 2002 split from Spears. He also told Watch What Happens Live host Andy Cohen that Spears occasionally still comes up in conversation.
"Now and then, yeah," the late-night host said.
"We used to hang out all the time, with Britney as well," he shared. "We met each other on Saturday Night Live. I remember going to a Super Bowl party at Planet Hollywood that Britney and Justin were having. It was just really funny."
Fallon confessed he never thought Timberlake and Spears would last longterm as a couple. The pair, who first met as preteens on The Mickey Mouse Club, called it quits in March 2002 amid rumors Spears had cheated.
"In the past, suffering has helped me compose songs. 'Cry Me a River' was written in a time of pain, like everyone knows," Timberlake said of his 2002 breakup anthem in an interview with Vogue Italia in 2016.