But does "The Tonight Show" host know the pop star better than Timberlake's own wife?
Fallon and Jessica Biel squared off in the "Best Friends Challenge" on "The Tonight Show" Thursday while Timberlake sat silently due to bruised vocal cords that have necessitated giving his voice a break.
Timberlake's wife and his BFF played a version of "The Newlywed Game" with flash cards to test their knowledge of Timberlake's favorite rappers, go-to cocktail and his ... safe word.
Biel jumped out to an early lead before both competitors raised some eyebrows when each of them correctly guessed "pineapple" as Timberlake's safe word.
"How the heck did that just happen?!" Fallon said. "That is so bizarre. Things just got weird."
Timberlake's final question asked what number he was thinking of between one and 5,000. Fallon nailed it while Biel walked off the set in frustration.
"I guess we really are soulmates,'' Fallon said. "We are best friends."