Mata, 37, has joined the Victory, where he is expected to get more game time under coach Arthur Diles, on a one-year deal.
The classy midfielder made just seven starts in 23 Wanderers appearances, scoring one goal.
"I am excited to be coming to Melbourne and to be part of one of the most respected clubs in the league and playing in front of the incredible members and fans," Mata said in a statement.
"I am driven and motivated to help this team win trophies, and to work with the club as part of a longer-term plan to strengthen football in the country."
Mata is the Victory's most high-profile recruit of an off-season signing spree, which has also included Socceroos Denis Genreau and Jason Davidson.
Coach Arthur Diles is attempting to go one better after reaching the grand final last year, which included a spiteful elimination final win over Mata's former team the Wanderers.
"We are incredibly fortunate to add someone of Juan's experience and character to the squad," Diles said in a statement.
"We are excited to welcome him into the changeroom, where we have no doubt his leadership and knowledge of the game will elevate the whole team.
"Juan's playing ambitions for this season and his long-term history of success with the game, align perfectly with the way we want to play for our chance to win trophies this season."
Mata's brilliant career before moving to Australia included winning the 2010 World Cup and 2012 Euros with Spain.
He won the Copa del Rey at Valencia in 2008 before moving to Chelsea, where he won the Champions League, FA Cup, Europa League and FA Community Shield between 2011 and 2014.
Mata spent nine years at United, where he won the Europa League, FA Cup, League Cup and Community Shield, scoring 51 goals in 285 appearances.
He then had short stints at Galatasaray and Vissel Kobe, winning the Super Lig and J1 League respectively before joining the Wanderers last September.
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