It seems unbelievable, but much like Canadian Prime Minister Justin Trudeau last year, Fallon's late to learn the lesson that doing blackface at any point in your past is something that won't stay secret.

After the 'SNL' clip from the year 2000 resurfaced early Tuesday, the hashtag #jimmyfallonisoverparty began trending on Twitter ... and now, the late-night talk show host is apologizing.

Fallon says, "I made a terrible decision to do an impersonation of Chris Rock while in blackface. There is no excuse for this."

He adds ... "I am very sorry for making this unquestionably offensive decision and thank all of you for holding me accountable."

In the sketch -- which we repeat, aired on TV in 2000 -- Jimmy comes out in blackface dressed as the famous comedian to join Regis Philbin, played by Darrell Hammond, and crack some Chris Rock-esque jokes.

Hard to say if people found it funny back then ... but even Jimmy agrees it definitely was NOT worth it.

Of course, Fallon hosts 'The Tonight Show' on NBC, which many folks online were eager to point out is the same network that fired Megyn Kelly in 2018 for making controversial comments defending the use of blackface.

No word yet from NBC on if there will be any consequences for Jimmy.