The 28-year-old joins the club following a successful trial period, having been a free agent after leaving English side Doncaster at the conclusion of the 2019/20 season.
Capped 13 times by the Ikale Tahi, Lokotui made his international debut against Japan in 2017 and was part of Tonga's 31-man squad for the 2019 Rugby World Cup.
Prior to linking up with the Warriors, the flanker had represented Counties Manakau with distinction in New Zealand's ITM (now Mitre10) Cup.
The Longoteme-born man scored six tries in 18 appearances for the Steelers, before heading north for an eye-catching 12-game stint with Doncaster in the English Championship.
"I'm really happy to sign for Glasgow," the flanker told glasgowwarriors.org.
"It's a great club, and I'm proud to be a part of the squad ahead of the new season.
"Everyone has made me feel really welcome - it's a great group of boys, and they've all made me feel right at home already. If I don't understand something that's been said, I know that all I need to do is ask them and they'll make sure I know what I have to do.
"I can't wait to prove myself to the fans, and give my best every time I run onto the field in a Glasgow jersey."
Glasgow Warriors Head Coach Danny Wilson added: "Fotu has really impressed us so far.
"He's strong over the ball and carries the ball hard into contact, and he's someone I think the fans will really enjoy watching.
"He's a very physical six or seven and with the current situation in the backrow we needed to add some depth.
"We're looking forward to seeing what he can do on the pitch in a Warriors shirt."
Meanwhile, Glasgow Warriors have announced that they will be supporting Bruce Flockhart as he takes an extended break from rugby. The club also confirmed that back-rower Jale Vakaloloma has left the club.