The 29-year-old's contract 'effectively commences in January 2016', the club revealed in a press release, and CEO Peter Filopoulos is hopeful of seeing the Republic of Ireland striker in action sooner rather than later.
"While our visa spots for the current season are full (at the maximum allowable limit of five) we have not given up hope that Andy Keogh could be playing football wearing a Perth Glory jersey in the 2015-16 season," Filopoulos said.
"We have until February 2 to register him within the January transfer window, should the opportunity materialise."
Keogh scored 12 goals in the A-League last season but was released by Glory after being caught up in the club's salary cap scandal.
"Once we became aware that Andy's playing stint at Thai Premier League Club, Ratchaburi had come to an end – we immediately made contact to get him back at the club and re-join his family in sunny Perth. We did not want to wait till the end of the season and risk losing him to another club," Filopoulos said.
Keogh admitted he was relishing a return to Glory after his short stint in the Thai Premier League.
"I've always said that I left Perth with a heavy heart. My stint in Asia didn't work out the way I intended, however I'm glad to have teamed up yet again with Kenny Lowe and the boys at Perth Glory," Keogh said.
"I am committed to helping Perth Glory become a powerhouse in the A-League and pledge to do my best to see it return to its glory days."