Feldman earned the nod over left-hander Brandon Finnegan, who will be starter in the second game, manager Bryan Price announced Monday.
Feldman will be opposed by right-hander Jeremy Hellickson, who was named the Phillies' Opening Day starter on Sunday.
The Reds signed the 34-year-old Feldman to a one-year deal on Jan. 26 to pitch either in the team's rotation or bullpen. He also started openers for the Texas Rangers (2010) and Houston Astros (2014).
"Baseball is one of those funny games, you never really know what's going to happen, you can't predict the future, so you just stay ready for everything," Feldman said Monday. "It's just an honor to take the ball on Opening Day and hopefully build off of it and have a great season."
Feldman was used mostly in relief last season, making only five starts. In 40 appearances with the Houston Astros and Toronto Blue Jays, he went 7-4 with a 3.97 ERA over 77 innings, with 56 strikeouts and 19 walks.
Feldman moved to the top of the rotation for the Reds after injuries to Homer Bailey and Anthony DeSclafani.
"His experience and the way he commands the strike zone I think really makes him the best candidate," Price said of Feldman. "It really comes down to him and Finnegan. My interpretation of Scott throughout spring training is that he's exactly the guy we were looking for and targeted -- a veteran guy with presence that commands the zone and has a veteran savvy that will serve us well. That will work well in an Opening Day environment."
Finnegan, who started 31 games for the Reds last year, gets the start in the second game on April 5. The 23-year-old went 10-11 with a 3.98 ERA over 172 innings.