The 32 year-old came through the Barcelona youth ranks and made his senior team debut in 2004 during a tour of France before spending the majority of his career in Spain's second tier and spells in Poland and Georgia.
"Andreu has spent time at clubs such as Barcelona and Malaga and we’re excited to add this sort of football pedigree to the playing squad," Western Sydney Wanderers CEO John Tsatsimas said.
"It is important to get players of the calibre of Andreu to add a significant backbone to our squad in the middle of the park.
"We look forward to welcoming Andreu to our Club when he arrives for pre-season training in the coming weeks."
Central midfielder Andreu appeared in the 2011 FIFA Club World Cup during a guest stint with Auckland City and joined Santander in 2013 and helped the club win promotion to La Liga but it finished second bottom in season 2014-2015 and will be back in the Segunda Division.
Western Sydney Wanderers Hyundai A-League 2015-2016 squad
Goalkeepers: Dean Bouzanis, Andrew Redmayne
Defenders: Jonathan Aspropotamitis, Shannon Cole, Shayne D'Cunha, Brendan Hamill, Scott Jamieson, Scott Neville, Matthew Spiranovic, Nikolai Topor-Stanley
Midfielders: Kearyn Baccus, Mateo Poljak, Jacob Pepper, Andreu Guerao Mayoral
Attackers: Mark Bridge, Romeo Castelen, Labinot Haliti, Golgol Mebrahtu, Brendon Santalab, Jaushua Sotirio
Contracted NYL players: Daniel Alessi, Alusine Fofanah, Liam Youlley