The former Raith Rovers and Morton forward is the latest new arrival at Firhill after Darian MacKinnon (Hamilton Academical), Zak Rudden (Rangers), Jamie Barjonas (Rangers, loan) and Lee O'Connor (Celtic, loan).
Manager Ian McCall told the club website: "I’m thrilled to add Brian to the squad for the rest of this season and beyond. He is a proven goalscorer at both Championship and Premiership level and has been someone I have been chasing for quite a while."