Guerrero, 26, was part of the squad that won Spain's first World Cup last month, making three substitute appearances.
The central midfielder, who has 16 caps, joined Atletico in 2022 - winning the Queen's Cup last season.
"Irene is a World Cup winner whose quality is an excellent addition to our team," United coach Marc Skinner said.
We are delighted to bring a player of Irene's stature to Manchester United Women."
United, who finished second in the league last year, open the new WSL campaign at Aston Villa on Oct. 1.
Persons:
Spain's Irene Guerrero, Molly Darlington, Irene Guerrero, Guerrero, Spain's, Irene, Marc Skinner, Aadi Nair, Ken Ferris
Organizations:
Soccer Football, FIFA, North Harbour, Manchester United, Atletico Madrid, Super League, Atletico, Aston Villa, Thomson
Locations:
New Zealand, Spain, Auckland , New Zealand, Bengaluru