The 21-year-old will line up at outside centre for a Stormers team that will be led by new captain Siyamthanda Kolisi in the final match of the opening round.
Fullback Sarel Marais will also be playing in his first Super Rugby game for the Stormers and forms part of a back three which includes Cheslin Kolbe and Dillyn Leyds out wide.
Viljoen will be partnered by Damian de Allende in midfield, with the experienced Jano Vermaak forming an exciting halfback combination with playmaker Jean-Luc du Plessis.
Kolisi is joined in the loose trio by Nizaam Carr and Rynhardt Elstadt, whilst vice-captain Eben Etzebeth and Pieter-Steph du Toit will be in the second row and up front props Wilco Louw and JC Janse van Rensburg will flank hooker Mbongeni Mbonambi.
There are also two Stormers debutants on the replacements bench in the form of hooker Ramone Samuels and utility back Dan Kriel.
Stormers head coach Robbie Fleck said that after a tough but rewarding pre-season, his players are excited to show what they are capable of at Newlands.
"The start of a new season is always an exciting time and for us to play at Newlands makes the occasion all the more special.
"We are determined to show our faithful fans what we can do in what should be a cracking game on Saturday," he said.
Stormers: 15 Sarel Marais, 14 Cheslin Kolbe, 13 EW Viljoen, 12 Damian de Allende, 11 Dillyn Leyds, 10 Jean-Luc du Plessis, 9 Jano Vermaak, 8 Nizaam Carr, 7 Siyamthanda Kolisi (captain), 6 Rynhardt Elstadt, 5 Pieter-Steph du Toit, 4 Eben Etzebeth, 3 Wilco Louw, 2 Mbongeni Mbonambi, 1 JC Janse van Rensburg.
Replacements: 16 Ramone Samuels, 17 Oliver Kebble, 18 Frans Malherbe, 19 Chris van Zyl, 20 Sikhumbuzo Notshe, 21 Dewaldt Duvenage, 22 Dan Kriel, 23 Robert du Preez.
Date: Saturday, February 25
Venue: Newlands, Cape Town
Kick-off: 19.30 (17.30 GMT)
Referee: Jaco van Heerden (South Africa)
Assistant referees: Shuhei Kubo (Japan), Stuart Berry (South Africa)
TMO: Marius Jonker (South Africa)
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