Harewood, who has scored five goals in the Premier League this season after joining Blackpool in the summer, has seen himself drop down the pecking order in recent months at Bloomfield Road.
Barnsley boss Mark Robins told the club's official website: "Marlon is a great addition to the squad.
"He has scored five Premier League goals this season and is a proven quality player.
"I am looking forward to working with him for the rest of the season.
"I would like to thank Blackpool FC and Ian Holloway for letting him come to Barnsley and also (Tykes owner) Patrick Cryne for getting this deal done."