The comic book where Batman first swooped onto the scene has set an impressive milestone … netting its highest public sale since it first graced newsstands nearly 85 years ago.

Detective Comics #27 fetched a jaw-dropping $1.825 million on auction site ComicConnect — and TMZ can reveal this ain’t the buyer’s first rodeo, ’cause while this is the most expensive version of the comic in his collection, it’s not the first copy he’s purchased.

This savvy collector believed so much in the rarity and demand of Batman’s first appearance in comics back in 1939 … he recently shelled out over a half mil for a lower-grade version.

The mysterious buyer isn’t your typical celeb or public figure — instead, he’s a real estate and hospitality businessman with a passion for all things made in the “golden age” of comics.

As for why Detective Comics #27 is so coveted … well, it holds a special place in the hearts of comic book collectors … ’cause it’s where the Dark Knight made his grand debut and there are only about 100 copies known to exist.

The epic intro unfolds when Commissioner Jim Gordon invites his friend Bruce Wayne to assist in a murder investigation. But when another murder goes down, it’s none other than Batman who swoops in to take charge — and as the story progresses, the identity of Batman is revealed to be Bruce himself.