Of course, we're talking about James Corden ... the now-former late night talk show host, who's a bona fide Harry stan -- and who actually showed up in the flesh on Saturday to help HS cap off his long-running "Love on Tour" at the RCF Arena in Reggio Emilia, Italy.

The comedian was actually among just a handful of famous faces on hand to watch Harry perform -- although he was certainly the most recognizable. Others included Rob Stringer, Kid Harpoon, Ben Winston, and additional members of Harry's close circle of friends and family.

Now, as for whether James just looked on from the VIP area ... well, not exactly. There's video of the dude getting right in the mix among HS' many fans -- dancing his face off.

Indeed ... the guy was absolutely living it up/singing along to Harry's set -- again proving he's crazy about the guy's music, which he's made very clear over the years in sit-down interviews with the singer ... or even in "Carpool Karaoke." Corden loves him some Styles.

As you know, Harry's been touring for almost 2 years straight now ... having traveled the world and packing out stadiums and venues all over, just about everywhere he's performed.

His time on the road has been memorable. In addition to his many eccentric outfits -- and, at times, wardrobe malfunctions -- he's also ... had lots of crap thrown at him onstage (some of which has made contact), helped fans do baby gender reveals, trotted out tons of big-name guest acts to sing with him, gotten quite sick, won Grammys ... and, of course, spurred many people -- including security guards -- to boogie down on the floor.

Offstage, he's made headlines as well ... including his relationship with Olivia Wilde -- which proved dramatic. And yes, his other romantic interests since have also turned heads as well.