Porthen, 21, has played a handful of games for the Stormers in the United Rugby Championship but is a former Junior Springbok and gets his chance after Neethling Fouche suffered a fractured arm.
"It’s great to see a young player like Zachary, who has emerged through our junior structures, come through the ranks into the senior set-up, and we are excited to see what he can do at this level," coach Rassie Erasmus said.
The Springboks take on Japan at Wembley Stadium in London on November 1 before travelling to Paris to meet France a week later in a repeat of their 2023 Rugby World Cup quarter-final.
They tackle Italy in Turin on November 15 before the latest instalment of their burgeoning grudge match with Ireland in Dublin on November 22.
They finish the tour against Wales in Cardiff a week later.
The group is largely the same as that which lifted the Rugby Championship trophy this month, though flanker Ben-Jason Dixon, hooker Johan Grobbelaar and prop Gerhard Steenekamp make their first appearances in the squad this campaign.
Lock Jean Kleyn as well as wings Arendse and Edwill van der Merwe return after injury.
Captain Siya Kolisi is likely to bring up a century of caps on tour as he sits on 98, which would make him the ninth Springbok to reach triple figures.
World Cup winning fullback Willie le Roux and scrumhalf Faf de Klerk have not been selected.
"We have delivered a few good performances but also a few disappointing ones this season, so this will be a good opportunity to test ourselves and see if we can build consistency from week to week," Erasmus said.
"It is going to take a massive effort, and we are excited about the tour."
South Africa squad:
Forwards: Lood de Jager, Ben-Jason Dixon, Pieter-Steph du Toit, Thomas du Toit, Eben Etzebeth, Johan Grobbelaar, Jean Kleyn, Siya Kolisi, Wilco Louw, Malcolm Marx, Franco Mostert, Ox Nche, Ruan Nortje, Zachary Porthen, Kwagga Smith, RG Snyman, Gerhard Steenekamp, Marco van Staden, Boan Venter, Jan-Hendrik Wessels, Jasper Wiese.
Backs: Kurt-Lee Arendse, Damian de Allende, Andre Esterhuizen, Sacha Feinberg-Mngomezulu, Ethan Hooker, Cheslin Kolbe, Jesse Kriel, Manie Libbok, Canan Moodie, Handre Pollard, Cobus Reinach, Morne van den Berg, Edwill van der Merwe, Damian Willemse, Grant Williams.