The 24-year-old Gibson has moved to Merseyside for an undisclosed fee, becoming the club's second signing of the January transfer window following Landon Donovan's loan move from Los Angeles Galaxy.
The midfielder, a product of the United youth team, scored 10 goals in 60 appearances for the Old Trafford side since breaking into the first team in the 2005-06 season.
Gibson, who was born in Northern Ireland but opted to play for the Republic of Ireland, has played 17 times for his country, scoring once.
Source: PA
Source: PA