The 36-year-old defender, who becomes the first Frenchman to play in the A-League, will be the Western Australian team's international marquee player after plumping for Perth over offers from Major League Soccer, the Far East and the Gulf.
Gallas, who spent 11 years in England with Chelsea, Arsenal and Tottenham, cited the club's forward thinking and commitment as his reasons for choosing them.
"After speaking to (manager) Alistair Edwards and (chief executive) Jason Brewer, the passion and ambition in the voices of both gentlemen enticed me to this new challenge in Australia," Gallas said in a statement.
"I look forward to wearing with pride the jersey of Perth Glory for this season.
"I hope to help my new team-mates and club achieve success in this season and hope to help deliver to the fans of Perth the championship they are looking for.
"I look forward to this new challenge and hope to excite the fans."
Edwards was equally happy about acquiring a player who has played under some of the word's most famous coaches.
"What better player in world football to bring to Western Australia than someone with 84 caps for one of the best footballing nations in the world?" he said.
"He has played alongside some of the best players in world football and worked under some of the most decorated managers and coaches of all time, including Arsene Wenger and Jose Mourinho, people whom I have the utmost respect for."
Brewer added: "William Gallas has played at the highest level in some of the most intense rivalries in the world, so the leadership he can provide to our up-and-coming younger players will be invaluable.
Gallas appeared in two World Cups for France, in which he was part of the team that were runners-up behind Italy in 2006 as well as in the squad that had a disastrous campaign four years later.
He also took part in the 2004 and 2008 European Championships, earning 84 caps for Les Bleus before retiring from international football in 2011.
Gallas also enjoyed a fine domestic career, starting his career at Caen where he was part of the Ligue 2-winning team before moving on to Marseille where he spent four years.
He then moved to Chelsea in 2001 and during a five-year spell won back to back Premier League titles as well as the League Cup.
However, in 2006 he left the Blues under a cloud for Arsenal where he spent four years until a move to north London rivals Tottenham before his release in June.