Hearts assistant head coach Stephen Frail said: "I hope Roman will gain valuable first-team experience at West Brom. We wish him well for the season ahead."
Bednar has not featured for Hearts this season due to a calf problem but he is expected to be fit after the international break.
Baggies boss Tony Mowbray told their official website: "Due to Roman's injury, he's not going to come straight into the team, but we're delighted to have him here. I think the fans will take to him because he has a great passion for the game. "Roman is a fighter who chases lost causes and can lead the line. When you ally that to his hunger for goals and his technical ability, he can be a real handful. During his early days at Hearts under George Burley, his robust style seemed to get him a goal most weeks."