Tom Welling Accidentally Posts Nude Photo on Instagram

Tom Welling apparently forgot one very important fact before posting a hotel-room pic to Instagram ... windows reflect!!!

The former "Smallville" star accidentally gave his 2.3 million Instagram followers an eyeful after sharing a photo of the New York City skyline from his hotel room ... with his nude reflection visible in the glass.

Welling -- who played Clark Kent on the hit superhero series -- captioned the Instagram Story "Hello NYCC" ... but fans quickly noticed the skyline wasn't the only thing in the shot.

The actor eventually deleted the post after it had been up for roughly 10 hours ... but that was apparently plenty of time for eagle-eyed followers to spot the NSFW slip-up and circulate screenshots online.

The 49-year-old is in the Big Apple for New York Comic Con, where he reunited with his "Smallville" castmates to celebrate the show's upcoming 25th anniversary. The series ran for 10 seasons from 2001 to 2011, following Clark's journey before he became Superman.

And here's the kicker ... Welling was apparently having a pretty good time at the convention despite the online commotion. He later posted another Instagram Story inviting fans to stop by and meet him.

Welling's "Smallville" costar Michael Rosenbaum took there on screen tension into real life, posting a matching nude selfie on his story poking fun at the naked slip-up ... and Superman even reposted the jab on his Insta.

We've reached out to reps for Welling, so far, no word back.