On December 14, 2007, Haren was traded to the Arizona Diamondbacks, along with Connor Robertson, for prospects including Carlos González, Brett Anderson, Aaron Cunningham, Greg Smith, Dana Eveland, and Chris Carter.
In his first season with the Diamondbacks he was selected to the 2008 All-Star game at Yankee Stadium along with his teammate, Brandon Webb.
He finished 2008 with a 3.33 ERA to go along with a 16–8 win–loss record, and a career-high 206 strikeouts in 216 innings.
His strikeout to walk ratio of 5.15 was the best in the National League, trailing only Roy Halladay
On August 6, 2008, Haren agreed to a four-year, $44.75 million contract with a team option for the 2013 season.
Haren was also selected as a 2009 NL All-Star, representing the Diamondbacks along with Justin Upton.
Haren finished the year with a record of 14–10, 3.14 ERA, and 223 strikeouts in 229⅓ innings, throwing three complete games in 33 starts.
He once again led the National League with a strikeout to walk ratio of 5.87, and ranked third in total number of strikeouts and ninth in strikeouts per nine innings (8.75 K/9).
His 1.003 WHIP was the best mark in the big leagues.

In 2009, he was named #33 on the Sporting News' list of the 50 greatest current players in baseball.
A panel of 100 baseball people, many of them members of the Baseball Hall of Fame and winners of major baseball awards, was polled to arrive at the list.
He began the 2010 season with a 7-8 record and a 4.60 ERA through July 25.
On December 14, 2007, Haren was traded to the Arizona Diamondbacks, along with Connor Robertson, for prospects including Carlos González, Brett Anderson, Aaron Cunningham, Greg Smith, Dana Eveland, and Chris Carter. In his first season with the Diamondbacks he was selected to the 2008 All-Star game at Yankee Stadium along with his teammate, Brandon Webb. He finished 2008 with a 3.33 ERA to go along with a 16–8 win–loss record, and a career-high 206 strikeouts in 216 innings. His strikeout to walk ratio of 5.15 was the best in the National League, trailing only Roy Halladay
On August 6, 2008, Haren agreed to a four-year, $44.75 million contract with a team option for the 2013 season. Haren was also selected as a 2009 NL All-Star, representing the Diamondbacks along with Justin Upton. Haren finished the year with a record of 14–10, 3.14 ERA, and 223 strikeouts in 229⅓ innings, throwing three complete games in 33 starts. He once again led the National League with a strikeout to walk ratio of 5.87, and ranked third in total number of strikeouts and ninth in strikeouts per nine innings (8.75 K/9). His 1.003 WHIP was the best mark in the big leagues.
In 2009, he was named #33 on the Sporting News' list of the 50 greatest current players in baseball. A panel of 100 baseball people, many of them members of the Baseball Hall of Fame and winners of major baseball awards, was polled to arrive at the list. He began the 2010 season with a 7-8 record and a 4.60 ERA through July 25.
Dan Haren