The D-backs on Tuesday acquired the left-handed Willis from the Tigers in exchange for right-hander Billy Buckner.
Willis' first start for Arizona will come Saturday against the Rockies at Chase Field.
Willis was designated for assignment by Detroit over the weekend. The Tigers also included cash in the deal to help offset Willis' $12 million salary this season. Detroit will pay his salary for this year minus the prorated portion of the minimum salary of $400,000.
"He brings a fresh outlook to our team and a fresh personality," said D-backs manager A.J. Hinch, whose team had dropped eight in a row heading into Tuesday night's game with the Dodgers. "We're a team that needs a little life at this point."
Willis, who was 1-2 with a 4.98 ERA in eight starts and one relief appearance for the Tigers this year, was signed to a three-year, $29 million contract extension by Detroit in 2007, not long after the Tigers acquired him from the Marlins in an eight-player trade that also netted Detroit first baseman Miguel Cabrera.
His two-plus seasons in Detroit were marked by injury and struggles with his command. So it's no surprise that Willis used the words "fresh start" numerous times Tuesday when discussing the deal.
"It was a great place to play baseball," Willis said of Detroit. "But I just didn't get it done. Sometimes for an athlete it's definitely rejuvenating that someone else wants you and believes in you. I was very excited. It's an opportunity for a fresh start with a team that plays a style very similar to the Marlins."
Florida is where Willis' finest moments came.
He burst on the scene with the Marlins in 2003, when he went 14-6 with a 3.30 ERA for the World Series champions. Quick with a smile and possessor of a magnetic personality, Willis quickly became a fan favorite while winning the National League Rookie of the Year Award.
Willis' best season was 2005, when he was 22-10 with a 2.63 ERA.
"I think he's always been known as a guy with a big personality," D-backs general manager Josh Byrnes said. "We encouraged him to be himself and to breathe some life into this team, which we need."
Willis, 28, won a job in Detroit's rotation this spring and had quality starts in three of his first four outings. But he struggled in May, going 0-1 with a 6.52 ERA, and the Tigers elected to remove him from the roster to make room for Max Scherzer, the former D-backs right-hander who was recalled from the Minors on Sunday.
Command became a serious issue for Willis in 2008, and he spent much of the season on the disabled list with forearm and knee injuries. He started last season on the DL with what the club called an anxiety disorder.
"I think I got to a point where it got to be a job," Willis said of his struggles. "You always try to be the best at what you do, but you really have to enjoy what you do. I just want to get back to that and really just go out there and enjoy the game of baseball."
Willis will get to live at home this summer, as he recently purchased a house in the Phoenix area. He fell in love with Arizona this past offseason while working out there at Athlete's Performance last offseason.
"I'm looking forward to getting out West in some of that good weather and playing some good baseball," Willis said.
Willis will become the first left-hander on the Arizona pitching staff since Jordan Norberto was optioned to Triple-A a couple of weeks into the season.
The D-backs are certainly aware of Willis' command issues, but they feel he is worth the risk.
"That could be an issue, but again, unfortunately the spot that we're in, we're hoping that a change of scenery is helpful," Byrnes said. "Again, he's got stuff that can play in the strike zone and coming back to the National League and getting into the flow of things, hopefully he can pitch like a lot of our starters have and give us a chance to win games. There's some progress with the strike throwing; it still needs to get better."
Buckner, 26, was originally a second-round Draft pick of the Royals in 2004 and was acquired by the D-backs in December 2007 for infielder Alberto Callaspo.
While he pitched well for the D-backs in September last season, Buckner was unable to win a spot in Arizona's rotation this spring. After a stint in Triple-A, Buckner was recalled in May and went 0-3 with an 11.08 ERA in three starts.
"Billy's worked hard," Hinch said. "It just didn't work here for him."