The One Direction star appeared on Saturday Night Live over the weekend and performed two tracks - new single Sign the Times and Ever Since New York.
The latter song has been accused using the same guitar riff as originated in a Badfinger song.
"Baby Blue by Badfinger, same bass line as Harry Styles new song (which is lovely but that was distracting)?" tweeted one viewer.
"I wonder if Harry Styles has ever heard Badfinger's Baby Blue, and if he will hear from their lawyer," another tweeted.
OK! Online has contacted Harry's reps for comment.
In a recent interview with Rolling Stone magazine, Harry admitted to loving older music, though wanted to ensure his new album wasn't compared to it.
"A lot my influences, and the stuff that I love, is older," he told Cameron Crowe. "So the thing I didn't want to do was, I didn't want to put out my first album and be like, 'He's tried to re-create the Sixties, Seventies, Eighties, Nineties.'
"Loads amazing music was written then, but I'm not saying I wish I lived back then. I wanted to do something that sounds like me. I just keep pushing forward."
Harry also discussed the meaning behind his debut solo single, Sign the Times, which he says was written from the point view a mother dying in childbirth.
The single debuted at number one in the UK and number four in the US.