The 36-year-old former Tottenham, Chelsea and Arsenal defender had considered a switch to USA's MLS but opted for a move down under after being impressed by Glory's proposal.
"After speaking to (Glory coach) Alistair Edwards and (CEO) Jason Brewer, the passion and ambition in the voices of both gentlemen enticed me to this new challenge in Australia," Gallas told Fox Sports Australia.
"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."
Negotiations between the former Tottenham defender and Glory took weeks but Brewer was thrilled to finally get his man.
"I'm absolutely delighted that Perth Glory has been able to attract someone of William Gallas' quality," he said.
"It demonstrates the growing reputation of the Hyundai A-League and its ability to attract world-class players.
"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 is the latest big-name signing to play in the A-League in recent seasons, joining former international stars Alessandro Del Piero, Shinji Ono, Harry Kewell and Emile Heskey.