Scotland coach Gregor Townsend has made four changes for Sunday's game in Nice, which the side must win if they want to keep alive hopes of a quarter-final place.
Steyn, who scored four tries when Scotland beat Tonga 61-14 in a friendly in November 2021, replaces Darcy Graham following the 18-3 loss to South Africa in their group opener.
Chris Harris comes in at centre in the place of Huw Jones but will wear the No.13 jersey, with Sione Tuipulotu moving to 12, while Scott Cummings takes over from Grant Gilchrist at lock.
In the other change, prop Rory Sutherland stands in for Pierre Schoeman.
"We have to impose our gameplan and our fitness on Tonga, and show our skills in the final quarter of the match," Townsend said.
"They are strong at the breakdown and we know they are going to be a massively physical threat at the set-piece."
Steyn is part of a back three with fellow South African-born winger Duhan van der Merwe and fullback Blair Kinghorn, while the mercurial Finn Russell will lead the backline from five-eighth. Ben White keeps his place in the No.9 jersey.
George Turner is at hooker and will have Sutherland and Zander Fagerson either side of him in the front row of the scrum, while Richie Gray keeps his place to start at lock alongside Cummings.
Captain Jamie Ritchie is at flanker with Rory Darge, and No.8 Jack Dempsey is the final member of the back row.
Scotland have won four of their five previous meetings with Tonga, all by more than 20 points, but were beaten 21-15 in Aberdeen in 2012.
SCOTLAND LINE-UP TO FACE TONGA:
Blair Kinghorn, Kyle Steyn, Chris Harris, Sione Tuipulotu, Duhan van der Merwe, Finn Russell, Ben White, Jack Dempsey, Rory Darge, Jamie Ritchie (capt), Scott Cummings, Richie Gray, Zander Fagerson, George Turner, Rory Sutherland. Res: Ewan Ashman, Pierre Schoeman, WP Nel, Sam Skinner, Matt Fagerson, George Horne, Huw Jones, Darcy Graham.
