Gallas, 36, has won 84 caps for France and played in the 2006 and 2010 World Cups and the 2004 European Championships. He had been without a club since ending a three-season stint with English Premier League side Tottenham Hotspur in July.
"I'm absolutely delighted that Perth Glory has been able to attract someone of William Gallas' quality," Perth CEO Jason Brewer said in a statement of the former Arsenal and Chelsea player.
"It demonstrates the growing reputation of the A-League and its ability to attract world class players."
The Frenchman joins the A-League at an exciting time of growth, with a 40 percent increase in attendances and 100 percent rise in TV viewers in the last campaign thanks to the arrivals of marquee names like Italy's Alessandro Del Piero, Japan's Shinji Ono and former England striker Emile Heskey.
The 2013-14 season kicked off earlier this month with Perth sitting ninth in the 10-team division after collecting one point from two games.
Perth head coach Alistair Edwards said the Frenchman's experience would be invaluable to his team.
"What better player to bring to Western Australia than someone with 84 caps for one of the best footballing nations in the world," he said.
"I am looking forward to sitting down with William over the next eight months to extract every piece of knowledge about his experiences... and whatever he can share about working with the greatest managers in world football over the past decade."