Arizona manager A.J. Hinch ended the suspense Friday, when he announced Haren would get the nod over Brandon Webb against the Padres on April 5 at Chase Field.
Hinch said he is targeting the third game of the season to be Webb's initial start with newly acquired Edwin Jackson starting Game 2.
Haren is coming off a 2009 that saw him go 14-10 with a 3.14 ERA in 33 starts and make a third straight All-Star Game appearance.
Webb, who has started the past four openers for the D-backs, tossed only four innings last year due to shoulder problems.
"Obviously, Haren had a spectacular season last year, got Cy Young votes, was a bright spot of last season and is very worthy of an Opening Day start," Hinch said. "On top of that, Brandon Webb returning to form is a topic that I'm going to have to talk about quite a bit this spring. By naming Danny as the Opening Day starter I get to give Webb a few extra days of Spring Training."
Webb had shoulder surgery in August and has been throwing off a mound for a couple of weeks. By starting Webb in the third game -- and assuming the D-backs skip their fifth starter the first time through the rotation -- Webb would get an extra day of rest before his second start.
"In a perfect world, ideally he would start the third game of the year based on how the off-days work in April," Hinch said of Webb. "So I think we can develop a plan for him to be able to buy a few extra days in April, to give him extra rest when necessary and I think the easy slot would be the third game of the year. A lot still has to happen between now and then."
Neither Webb nor Haren were at the complex during the time reporters were present Friday. They are expected to be there Saturday for the team's first workout.
Haren has started Opening Day on one other occasion. In 2007 while pitching for the A's, he allowed four unearned runs in six innings in Oakland's opener in Seattle.
Although Haren will get the first start, Hinch made it clear he does not have a No. 1, No. 2 or No. 3 pitcher.
"Opening Day is one day," he said. "I'm not going to put numbers by [the pitchers]."
Randy Johnson holds the club mark for most openers started, having pitched each one from 1999-2004.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.