The 24-year-old arrived in Scotland at the weekend after two seasons in New Zealand with North Harbour.
The 6ft 5in, 115kg Fifita has also played in Spain, with the El Salvador club, helping them win the División de Honor title – the top tier of Spanish rugby - in 2018-19, scoring 13 tries in 16 matches.
He made his international debut earlier this year, coming off the bench in Tonga’s defeat to New Zealand in Auckland before earning a second cap against Samoa in a Rugby World Cup qualifier.
Scotland are due to play Tonga later this month, with the islanders scheduled to face Gregor Townsend’s side in their first match of the Autumn Nations Series at BT Murrayfield on October 30.
Fifita will become the fifth Tongan – and first Tongan back - to represent Glasgow Warriors, following in the footsteps of Ofa Fainga’anuku, Sila Puafisi, Siua Halanukonuka and Fotu Lokotui.
Glasgow Warriors coach Danny Wilson said: “We’re really excited to be able to bring Walter into our squad.
“He’s a big, abrasive player who we believe can be a real asset to us out wide.
“We’re looking forward to fully integrating him into our environment ahead of the next block of fixtures, and we’re looking forward to seeing him perform in front of a packed out Scotstoun crowd.”
Fifita told glasgowwarriors.org: “I’m pretty stoked to be sign for Glasgow and start this new chapter in my career.
“I can’t wait to step out onto the field with these guys and I’m really happy.
“I’m really grateful for the opportunity and I want to thank both my agent and Glasgow for presenting me with it. Coming overseas was a big chance for me – I enjoyed my experience of it in the past when I was in Spain, and I’m looking forward to making more memories.
“I love to carry the ball and hopefully I can show the fans what I can do on the field.”
Glasgow have also been linked with a move for Sale Sharks’ South African lock JP du Preez.