Cardinals manager Mike Matheny told Martinez about the choice before he pitched Wednesday in an exhibition game against Washington.
The Cardinals host the NL Central rival Cubs on April 2.
Adam Wainwright had started the past four openers for the Cardinals. Matheny said Wainwright took the news well about Martinez getting the assignment this year.
The 25-year-old Martinez was an All-Star in 2015. He went 16-9 with a 3.04 ERA last year.
