The decision was announced on Monday morning by manager Fredi Gonzalez.
Asked why it was Willis, Gonzalez joked: "It landed heads. ... Come on.
"He's the guy who has been here the longest. He's the leader of that pitching staff. He's the guy we want there for the matchup."
Following Willis in the rotation will be Scott Olsen, Anibal Sanchez and Ricky Nolasco, who will start the home opener on April 6 against Phillies at Dolphin Stadium.
Because of an off day on April 5, the Marlins will go with a four-man rotation until April 10. The front-runner for the fifth spot is Sergio Mitre, while Yusmeiro Petit is also in consideration.
Gonzalez opted to go with lefties in back-to-back games in Washington to open the season, rather than split them up with Sanchez as the No. 2 starter.
"For me, there was no reason to split them up," Gonzalez said.
By going with Willis and Olsen as Nos. 1 and 2, those two southpaws will fall in line to face the Nationals, Phillies, Braves and Mets in April. Gonzalez said that the start of the season is the one time when a rotation can be set up to match up starters against specific teams.
Barring weather delays or anything else unexpected, the Marlins can use lefties against their National League East Division rivals. The Phillies feature All-Star lefty hitters Chase Utley and Ryan Howard, while the Mets have Carlos Delgado, along with switch-hitters Jose Reyes and Carlos Beltran.
The rotation for the April 6-8 series with the Phillies will be Nolasco, Willis and Olsen.
Thus far, Willis is having an up-and-down spring in terms of statistics. His record is 0-3 with a 6.92 ERA in four starts. One of his objectives now is lowering his walk totals, and he is encouraged that he has only issued just two free passes in 13 innings this spring. In 2006, Willis walked a career-high 83 batters, and he led the Major Leagues by hitting 19 batters with pitches.
Last season, Willis got the Opening Day nod and tossed five scoreless innings in a no-decision against the Astros at Minute Maid Park. The Marlins ended up losing, 1-0, that day.
"Last year was cool, because you just saw the jets flying overhead and the little red, white and blue banners," Willis said. "It was pretty cool being part of it. Everything is a little different on Opening Day. Lineup cards are a little different."
One thing Willis hopes to do differently this Opening Day is to toss first-pitch strikes. His first pitch in '06 sailed behind Houston's leadoff hitter Craig Biggio.
The 25-year-old Willis is coming off a 12-12 year with a 3.87 ERA. He made 34 starts and logged 223 1/3 innings.
Since being called up from Double-A in May 2003, Willis is 58-39 with a 3.44 career ERA. His 58 victories are the most by a Marlins pitcher.
Signed to a one-year, $6.45 million contract, Willis is arbitration-eligible through the 2009 season.
The left-hander is a two-time All-Star, and in 2003, Willis was the National League Rookie of the Year. In 2005, when he went 22-10 and topped the Major Leagues in victories, Willis also became the first 20-game winner in Marlins history.
"He's a leader. He's a manager's dream," Gonzalez said. "You do not have to worry about him at all, whether it is on the mound or the other four days. He comes in and works. He gets his work done, and everybody follows him. He leads by good examples."