Mariners manager Lloyd McClendon made it official on Saturday.
Hernandez will pitch April 6 in Seattle against the AL West champion Los Angeles Angels.
This will be the eighth career opening-day start for Hernandez. He is 5-0 with a 1.52 ERA in previous openers.
McClendon had previously said he was "99.999 percent" sure Hernandez would be the starter, and now has made it 100 percent certain.