The actor's smash-hit series, "Baby Reindeer," draws from his real-life encounters with a female stalker -- who goes by the name Martha in the show to shield the actual person's identity -- and fans have been doing their own deep dives in trying to dox the real McCoy.

Internet sleuths have gone down the rabbit hole trying to unmask Martha's real identity, as well as other characters -- including one featured in the show who's depicted as grooming/raping Gadd ... which has led to one particular man being falsely implicated.

This poor guy was getting bombarded by people who'd seen the show and felt he fit the profile of the so-called "groomer" ... and now, Gadd's coming out to defend the man.

He hopped online and wrote, "People I love, have worked with, and admire (including Sean Foley) are unfairly getting caught up in speculation. Please don’t speculate on who any of the real life people could be. That’s not the point of our show."

Accusations against Foley spiraled so out of control ... and it's gotten so bad, he actually says he's gotten cops involved due to defamatory and completely false accusations he's been facing from 'BR' viewers ... who believe he's the one who sexually abused Gadd.

In case it wasn't already clear ... Gadd reenacts things in the show that he says happened to him in real life -- which is why folks are in a frenzy trying to find the actual culprits.

Anyway, the actress who plays Martha in 'Baby Reindeer,' Jessica Gunning, has also chirped up, telling BBC ... "If you like the show and you are a fan of it, you should stick with the story of Martha and Donny being what connects you, not trying to do any detective work and find out any real identities."