'Tonight Show' MC Apologizes to BTS for Joke Deemed Racist by Fandom

"The Tonight Show Starring Jimmy Fallon" is in the crosshairs of BTS fans for a joke made by the MC during Wednesday's taping ... and he's since apologized to the international superstar group.

Here's the deal ... Seth Herzog, the show's warm-up comic, and Jimmy Fallon have been getting flamed on social media after audience members said they felt a joke made at the taping was racist toward the globally popular band ... and fans demanded an apology.

A source with direct knowledge tells TMZ ... Herzog apologized to BTS for the situation ... and network brass spoke to Herzog about the incident.

We're told the joke in question was, "Anybody here from the North? No? Nobody?" in reference to North Korea. The quip was meant to be a riff on the "Tell me where you're from" premise ... but some BTS fans in the audience thought it was targeted at the band, and blasted the MC on X. From there, the BTS fandom went into full attack mode on the show and Jimmy.

BTS' appearance on 'The Tonight Show' will be spread out over two days, Wednesday and Thursday ... featuring interviews and musical performances.