The 24-year-old joins from Athletic Bilbao, where she scored 63 objectives in 161 appearances north of six years.
Garcia is United chief Marc Skinner’s fifth getting paperwork done for the new Women’s Super League season, what begins on 9 September.
“The way that a club the size of Manchester United has placed their confidence in me is a little glimpse of heaven,” she said.
Garcia, who played for Spain at this late spring’s European Championship, added: “I as of now feel especially at ease here. There’s an extraordinary family air at the club, which is the specific climate wherein I flourish.
“I will give all that I have on the field to do right by our fans and ideally score a couple of objectives too!”
Joined have likewise acquired Maya Le Tissier, Grace Clinton, Rachel Williams and Canada forward Adriana Leon throughout the mid year move window.