The 36-year-old, who won 84 caps for France and played at two World Cups, is expected to arrive Down Under ahead of Perth's round five tie against Wellington.
"After speaking to Alistair Edwards and Jason Brewer, the passion and ambition in the voices of both gentlemen enticed me to this new challenge in Australia. I look forward to wearing with pride the Jersey of Perth Glory for this season," Gallas is quoted as saying in a statement.
"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," added the ex-Chelsea man.