The Spain veteran, who won Euro 2008 with La Roja, has left Villarreal for the USA's second division, becoming part of coach Giovanni Savarese's relaunched franchiseMarcos Senna as they bid to earn promotion to the MLS.
Senna, a key member of Spain's 2008 European championship-winning side, left Villarreal after 11 seasons at the end of the previous campaign, having helped guide the Yellow Submarines back to the Primera division in Spain.
The 36-year-old has now joined the second-tier NASL team, which will take part in a full campaign next season for the first time since the original club collapsed in 1985.
“This is a very special moment for me in joining the Cosmos, because I face a new challenge, but at the same time it has been a difficult decision to leave Villarreal after 11 seasons,” Senna said in a statement on Thursday.
Senna, who played nearly 300 games for Villarreal from 2002, becomes the most noteworthy signing for coach Giovanni Savarese's side this summer.