Donald Zackary Greinke (born October 21, 1983 in Orlando, Florida is a Major League Baseball player for the Kansas City Royals. He was a leader for the Conway (Orlando, Florida) Little Leauge Senior World Series Champions in 1998. He also played for the United States Junior Olympic team in 1999. He went to high school in Apopka, Florida. He is a right-handed pitcher, who throws a fastball topping out at 99 mph (159 km/h); a slider; changeup; curveball; and a slow curve, which he has thrown as slow as 50 mph (80 km/h).

After being named the Gatorade National Player of the Year in 2002, he was selected out of high school (and turning down a scholarship to Clemson) in the first round of the 2002 draft, he spent the winter of 2002-2003 in the Puerto Rican League. Afterwards, he played for the Wilmington Blue Rocks and Wichita Wranglers, where he was named the Royals Minor League Pitcher of the year, and The Sporting News Minor League Player of the Year, with a 15-4 record and 1.93 ERA. Greinke made his major-league debut on May 22, 2004 against the Oakland Athletics, starting the game where Reggie Jackson's number was retired.

He ended the 2004 season with a record of 8 wins and 11 losses (second-highest on the team, after Jimmy Gobble) and a 3.97 ERA. Greinke has been compared to future Hall of Famer Greg Maddux and Royals great Bret Saberhagen. He has sports endorsement deals with Adidas and Rawlings. However, Greinke's 2005 season was not as successful, as he went 5-17 with a 5.80 ERA.

Pitching in the American League, Greinke rarely bats due to the designated hitter rule. Greinke's first major league hit was a home run off Arizona Diamondbacks pitcher Russ Ortiz in a 12-11 loss on June 10, 2005.

Greinke left the Royals spring training camp for personal reasons in late February, 2006. It was later revealed that he was suffering from social anxiety disorder and was having trouble working with others. He reported back to the Royals' spring training facility in Surprise, Arizona, on April 17th, where he has been undergoing pitching sessions. He was placed on the 60-day disabled list due to psychological issues and took time away from baseball entirely until returning on a rehab assignment with Wichita. In 2007, Greinke succeeded in staking his claim of the inside portion of the plate and was dominant throughout spring training. He returned to the Royals rotation at the start of the season, but was sent to the bullpen in early May after alternating good starts with bad ones and compiling a 1-4 record with a 5.71 ERA. As a reliever, he was "lights out", going 4-1 with a 3.54 ERA. After the 2007 season he signed a one year contract to stay in Kansas City and to avoid arbitration.

Zack's brother, Luke, plays baseball for Auburn University, and currently leads the team in hitting as an outfielder/designated hitter.