The midfielder joins United following spells with Real Betis, Levante and most recently Atletico Madrid.
This summer saw Irene’s Spain side crowned world champions at the FIFA Women’s World Cup, following a 1-0 victory over England in the final on 20 August.
Irene Guerrero said: “I feel very happy and very privileged to be here.
“I am very proud to be part of the Manchester United family and look forward to achieving all our goals together.”
Marc Skinner commented: “Irene is a World Cup winner whose quality is an excellent addition to our team.
“The experience she brings, both domestically and internationally, will be key as we pursue success across the multiple competitions we face this season.