But just how well do these pals really know each other? On Wednesday's "Tonight Show," they put their friendship to the test by taking the Best Friends Challenge.
Fallon kicked off the match, which resembled a friends-only version of "The Newlywed Game," by pulling out a box covered with photos of history's greatest BFFs — Bert and Ernie, C-3PO and R2-D2, and Tina Fey and Amy Poehler.
All he and JT had to do to prove they deserved a spot alongside those dynamic duos was to answer a few questions correctly.
Of course, some questions were easier than others.
For instance, when asked what their preferred mode of transportation was, they both wrote "tandem bike." (And yes, they've actually joined forces for double-pedal power in the past.)
But when the topic of Timberlake's favorite TV show as a kid came up, they were divided. The right answer was "Cheers," but Fallon had him pegged as more of a "Gilmore Girls" boy.
Everything came down to the final question: What song do they always sing together?
These winners are undeniable besties, but we really didn’t need any proof. Their constant on-screen antics — including this brand-new installment of their Camp Winnipesaukee gag — had us convinced ages ago.
Timberlake also told the TV host that he recently voted — "super quiet, under the radar," he joked, referencing a possibly illegal selfie he took in a Tennessee voting booth.
"Don't take a picture of yourself in there," he deadpanned to the camera. "I had no idea."
But as it turns out, he's in the clear. Whew!
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