Guerrero, 26, featured for Spain three times on their run to World Cup victory.
After finishing runners-up in the last WSL season, Manchester United will be hoping Guerrero, who signs from Atletico Madrid, can help them go one better this season.
Manager Marc Skinner said: "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."
Guerrero added: "I am very proud to be part of the Manchester United family and look forward to achieving all our goals together."
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She is Manchester United's sixth signing this summer, as they attempt to make up for the loss of Lionesses striker Alessia Russo.
Other signings include World Cup Golden Boot winner Hinata Miyazawa and Barcelona's Champions League winner Geyse Ferreira.
Everton defender Gabby George and American goalkeeper Phallon Tullis-Joyce are reported to be their key targets before the transfer window ends.
The WSL kicks off on Sunday, October 1 when Manchester United will face Aston Villa at Villa Park.