I once had a friend tell me that he doesn’t respect Steve Martin because Steve Martin is Spanish (aka Latino) and doesn’t “admit” it. Of course he was thinking of this as “passing”; I don’t feel strongly about that but it did pique my curiosity about Martin. I can’t find any evidence one way or another. Any clues?
Never heard anything like that. FWIW, his IMDB Bio suggests nothing of the sort.
Wikipedia, and I don’t know where I’d go for more reliable info, says his family is entirely Anglo (English, Irish, Scottish and Welsh ancestry). He goes by his real name; he hasn’t shortened it from Martinez or anything. I’m pretty sure your friend is completely wrong.
Maybe he’s Spanish now but he was born a poor black child.
(His hometown is Waco, Texas.)
… or maybe the person meant Ricky Martin?
Well, he was in The Spanish Prisoner.
… or Cheech Marin? Yeah, I think he is Spanish!
I remember there was some question as to wether Steve Martin was Esteban, the Spanish Guitar Hocking Informercial Dude.
Nope, Esteban Martinez isn’t the slightest bit Hispanic.

Maybe your friend was thinking of Martin Sheen, one of whose actor sons (Emilio Estevez) reclaimed their original Latino surname.
The friend may be thinking of “Jorge Martín”, played by Andrew Rubin in the first Police Academy movie. About midway through, he confesses to Mahoney (Steve Guttenberg) that his real name is George Martin and he fakes being Latin to get women.
It’s a stretch, I know.
Seriously?! Had no idea those two were related. Martin and Charle and Emilio.
Wow.
Of course, they really don’t look it.
Gee, he seemed pretty authentic in the ¡Three Amigos!… 
Didn’t you always wonder why Martin and Emilio did all those movies together?
Martin and Charlie as father and son: Wall Street, No Code of Contact
Martin and Emilio as father and son: In the Custody of Strangers
Martin and Emilio as the same character at different ages: Nightbreaker
Charlie and Emilio as brothers: Rated X
And cameos in each other’s movies are common.
The only Emilio Estevex movie I saw was the Mighty Ducks. And maybe some movie about them being hunted by some guys in L.A. Or something. He mostly retired by the time I was old enough to watch his flicks.
Don’t forget Men at Work, possibly the best comedy ever about Peeping-Tom-Garbagemen-Entangled-With-Mob-Hitmen-In-A-Toxic-Wast-Dumping-Scheme. 
I omitted it because the imdb.com age says their characters have different surnames and I was going for a list in whch Martin, Charlie and Emilio played related characters.
[sub]I loved you in Wall Street![/sub]
Don’t forget Uncle Joe (who has an amazingly long list of straight-to-videos in his filmography).