Tillman, 25, went 16-7 with 3.71 ERA with 179 strikeouts in 206.1 innings last season and was selected as an All-Star for the first time. Tillman set career bests in innings pitched and wins.
"I'm proud of him. He deserves it," Showalter said.
Showalter said veteran Ubaldo Jimenez will start the second game against the Red Sox. Tillman will be the fifth different Opening Day starter in the last five years. Last year, Jason Hammel started on Opening Day for the team.
"Anytime you get the chance to get that kind of opportunity it's a big deal," Tillman said.SI WIRE: Orioles 3B Manny Machado could miss Opening Day after injury setback