In a repeat of their 2018 fight, the Alabama fighter soaked up pressure from his older opponent before sending him to the canvas with a straight right hand to the forehead.
Ortiz was on his feet before the end of the count but was still shaky and the referee called an end to proceedings.
The decision was a tough one for the 40-year old Cuban, who had made all the running in the early rounds.
It takes Wilder’s record to 42-0-1 and sets him up for a February, 2020 date with Tyson Fury, the undefeated British boxer he drew with in Los Angeles almost a year ago.
“After that I am looking for a unification bout,” Wilder said. “I want one champion, one face, one name and he goes by the name Deontay Wilder.”