The 31-year old former Melbourne Victory striker will head to Washington next month to take up an opportunity as one of the few Australians ever to play in the American league and is looking forward to the challenge.
"I'm delighted to be joining such a successful club as D.C. United," said Allsopp. "Their history speaks for itself and I'm really excited about the move.
"Playing in MLS is a challenge and I am really looking forward to it. I'm hoping to make a real impact to help carry on the club's great tradition."
Allsopp has been capped three times at international level with the Socceroos and will now show his wares in the one of the fastest-growing leagues in world football after spending the past few months in Qatar with Al-Rayyan.
The 2010 MLS season commences in March with DC United taking on the Kansas City Wizards away on Saturday the 27th.