"We feel that either one of these cities will create a world-class event by providing beautiful and challenging courses and outstanding hospitality for our athletes and their families and friends."
Ironman officials are visiting the two locations over the next week as the selection process continues.
The Ironman 70.3 World Championship began an annual global rotation in 2014 in Mont-Tremblant in Quebec, Canada. The 2015 race was held in Europe for the first time, in Zell am See-Kaprun, SalzburgerLand, Austria.
In 2016 it shifted to the Southern Hemisphere for the first time to Queensland, Australia. In 2017, the world championship returned to the United States with the host city Chattanooga in Tennessee, where it first became a two-day event.
Port Elizabeth in Nelson Mandela Bay will host this year's edition on September 1 and 2.
Next year the event will move to Nice, France.
Prior to 2014 it was held in Henderson, Nevada and Clearwater, Florida.
Qualification for the 2020 Ironman 70.3 World Championship will begin in July 2019 with about 190,000 registered athletes from around the world expected to compete at any of the more than 120 events in the global series.