The striker, who has been on trial at the club, will join from Angolan side Petro Athletico on a three-year deal.
"We have had Manucho here for a three-week trial and have been impressed enough to offer him a contract," said boss Sir Alex Ferguson.
"He is a tall, agile, quick forward and through contacts that Carlos (Queiroz) has, was brought to our attention around six months ago."
606: DEBATE
Manucho, whose real name is Mateus Alberto Contreiras Goncalves, arrives at Old Trafford in January subject to a work permit.
He joins Carlos Tevez, Wayne Rooney, Louis Saha and Dong Fangzhou in United's striking department.
Ferguson has ruled out another move to tempt Swedish striker Henrik Larsson back to Old Trafford.
The 36-year-old Swede scored three goals in 13 games during a successful three-month loan 12 months ago and resisted the chance to stay longer, keeping to his promise of returning to home-town club Helsingborgs.
"Henrik promised his family back then he would go home," Ferguson added.
"He honoured it, and we honoured it. It would be an interesting one, obviously - but I don't think so.
"At this time of year, it is always difficult to get the players you would like to get.
"You look at the criteria of playing in European football. That is not easy."
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