The versatile player, whose glittering club career has included lengthy spells at Valencia, Chelsea and Manchester United, moves to Wanderers after leaving Japan's Vissel Kobe in January.
"I'm really excited to join the Western Sydney Wanderers ... and I cannot wait to arrive and train with the team to prepare for the season ahead," Mata said in a club news release.
"I'm feeling grateful and excited for this new adventure, and looking forward to trying to win another championship in a different league."
Wanderers are the only Australian club to have won the Asian Champions League, which they did in 2014 only two years after being founded as an expansion team.
Their fierce cross-town rivals Sydney FC announced the signing of former Brazil winger Douglas Costa on a two-year deal last week.
The 2024/25 A-League season starts in mid-October.
