The Denmark international moves to Craven Cottage until the end of the season.
"I'm very pleased to have got the deal done and I'm looking forward to contributing to this fantastic Club," he told the club's official website.
"Of course I've heard a lot of good things from Brede and it's been a target of mine to come here so I'm delighted to be here really.
"The Premier League is the biggest league in the world. I've played in the Bundesliga which is also a really nice league, but this is the biggest challenge you could get as a footballer - this is the motherland of football."