Dempster (17-6, 2.99 ERA) will make his final tuneup on Friday against the Milwaukee Brewers, which will kick off the last regular-season series for the Cubs. Chicago opens the playoffs next Wednesday at Wrigley Field. The Cubs' opponent has yet to be determined.
The right-hander didn't want to comment about the announcement before Thursday's game against the New York Mets because Piniella hadn't said anything yet.
"I have to pitch tomorrow," Dempster said. "I want to be ready and end the season on a good note and get ready for the playoffs."
Named to his second All-Star Game this year, Dempster ranks among the NL leaders in ERA, wins and strikeouts (183). He worked hard this season to earn a spot in the rotation.
"It's been a fun year," Dempster said. "I'm hoping to be pitching late into October sometime, maybe right before Halloween. That would be kind of cool. Whatever the situation, I'm looking forward to it."
Piniella could've chosen Opening Day starter Carlos Zambrano, who did not get a decision on Wednesday night against the Mets in what was likely his last regular-season start, or lefty Ted Lilly, who has won a career-high 16 games.
"[Dempster] deserves it," Piniella said. "He's 14-3 at home, he's won more games than any [of the other Cubs] pitchers at home. He's been extremely consistent and reliable all season long. He'll be our opening-game starter. From there, we have a little work to do.
"I feel comfortable with all the starters we'll pitch in the postseason. I don't think you can go wrong with any selection. Dempster's our 17-game winner, Games 1 and 5 are at home, so it makes all the sense in the world."
Dempster agreed any of his rotation mates could've gotten the call.
"You have a nice pick of the litter to chose anybody," Dempster said. "I've always said one of the biggest things about our team is the depth of our team."
"He's a tremendous human being and teammate," Cubs general manager Jim Hendry said. "Nobody's smart enough to predict he'd go out and pitch in the All-Star Game and win 17 games, but there wasn't anybody betting against him about being good.
"It's not like we sat around all winter and thought, 'Oh no, we're in trouble because he can't do it.' We all thought he'd be in the rotation. He's that kind of guy. I knew when he left my office last October, I knew he'd be on a mission to get himself 110 percent ready. My hat's off to him."
Dempster had seasons of 33, 24 and 28 saves for the Cubs in his comeback from Tommy John elbow reconstruction surgery. He had been a starter for the Florida Marlins and Cincinnati Reds. The Cubs signed him as a free agent, and he's paid huge dividends.
"He's just a winner, and he makes everybody around him better," Hendry said. "He's one of the leaders of the club. He walks the walk, that's what he does, that's what he's all about. I love the guy."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.