The former Arsenal and Chelsea defender had been without a club since the end of last season after his contract expired at Tottenham Hotspur.
The French international was linked with a move to the MLS, but Gallas insists he is impressed by Glory's plans.
"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 teammates 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."