Webber, 27 was released by the Blades this summer; and because he was without a club the South Coast club were able to complete a full transfer despite the transfer window shutting a week ago. Over the summer he had been subject to moves to Middlesbrough and Swansea City.
He joined the Blades initially on loan from Watford in March 2005 until the end of that season before making a £500,000 transfer - he netted inside two minutes on his debut as United recorded a 4-0 victory over Leeds United at Elland Road.
During his time at Bramall Lane the striker made a total of 80 (+45) league and cup appearances whilst finding the back of the net 26 times.
He joins the Fratton Park club on a two year deal.