Lee initially claimed that he did not have any involvement with the film back in January, reported Variety.
The former Marvel Comics chairman revealed that he has recently filmed his role while appearing at the Chicago expo c2e2.
When an audience member at the expo questioned whether he would shoot a scene for the comic book movie, Lee answered, "I did, already."
The comics legend remained coy over what his cameo will entail.
"I can't tell you what it is, but I can tell you I do not understand what it was or why I did it. It has me with a girl - a very pretty girl - that's all I can tell you," he said.
Zoe Saldana, Chris Pratt, Dave Bautista, Vin Diesel and Bradley Cooper are part of the movie's core cast.
'Guardians of the Galaxy' is slated to open in US and UK theaters on August 1.