The Spain veteran has left Villarreal for the USA's second division, becoming part of coach Giovanni Savarese's relaunched franchise
Senna, a key member of Spain's 2008 European championship-winning side, left Villarreal after 11 seasonsat 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.