Mark Ruffalo has one less reason to Hulk out -- "The Avengers" star lost his wallet in Chicago, but it was returned by a total stranger ... who had no idea he was dealing with a big time celeb.
Ross McHale tells TMZ he found the wallet in a cab last Friday ... and saw Ruffalo's name on an ID, but didn't recognize the name so he Googled it.
McHale's clearly not fan of the Hulk, "Zodiac," or "54" ... because he says Mark's creds still didn't ring any bells. But he did get enough info to reach out on Twitter, where Mark eventually responded ... "Thank you! Wow! Another point for the decency in people."
Ruffalo didn't offer a reward, but Ross is hoping for a pay-it-forward kinda deal. He tells us ... "If [Mark] ever finds my wallet I expect him to return the favor."