The young Italian has not played competitively since February 2 after undergoing surgery on a niggling ankle injury.
Macheda has made just 12 starts over the past three seasons, spending time on loan at Sampdoria and Queens Park Rangers.
His spell at Loftus Road last season was cut short, but the 20-year-old is happy to be back at Old Trafford.
He told the club's official website: "It's great to be back at Manchester United. When you go on loan you really understand how important it is to play for this club.
"I'm really happy to be back and I can't wait for the start of the season.
"Last season was disappointing. In the first six months I didn't get a lot of opportunities here so we decided it was best for me to go on loan.
"When I got to QPR things went a bit wrong, though, as I was feeling my ankle a lot. But now I feel a lot better and want to start the season well. It's a big season for me."
Macheda is looking forward to being part of United’s pre-season tour of South Africa, China and Europe as he looks to step up his rehabilitation and prove his worth to manager Sir Alex Ferguson.
"I'm really looking forward to it," he added. "It's always great to visit fans in different countries. You meet new people, you see new cities, you experience different cultures.
"I didn't know how big the club was around the world when I went away for the first time in 2009. But I was very impressed by Malaysia, China and Korea.
"We have great support all over the world and I love being part of this team."
