On January 10, 2010, Chapman agreed to a long-term contract with the Cincinnati Reds. The Reds announced that they had signed Chapman to a six-year contract, worth $30.25 million according to MLB sources. The Associated Press reported that the bonus totals $100.25 million, paid annually over 11 years, with an additional bonus if he became eligible for salary arbitration in 2012 or 2013.
Chapman made his Major League debut August 31, 2010, in the eighth inning against the Milwaukee Brewers; his first pitch was clocked at 98 mph (158 km/h) as a called strike (which was promptly tossed to the dugout by catcher Ryan Hanigan, to be saved). In nine pitches he retired the side. He recorded his first Major League win on September 1 after pitching an inning of relief against the Brewers. Chapman threw the fastest pitch recognized by MLB on September 24, 2010, at Petco Park in San Diego, California. It was clocked at 105.1 mph to Tony Gwynn Jr. in the eighth inning.
With a three-quarters delivery, Chapman throws four pitches: a four-seam fastball averaging 99–100 miles per hour (159–161 km/h), a slider averaging 87–88 miles per hour (140–142 km/h), a splitter, and a change-up. His fastball averaged 100.3 miles per hour (161.4 km/h) in 2010, but that declined to 98.6 miles per hour (158.7 km/h) in 2011 and 97.8 miles per hour (157.4 km/h) through August 2012. This more modest speed might have been part of an attempt to better control his fastball. By 2015 Chapman's fastball averaged 99.98 miles per hour (160.90 km/h). That same year, Statcast revealed that Chapman threw the 62 fastest pitches of the season, topping out at 103.92 miles per hour (167.24 km/h).
On September 24, 2010, against the San Diego Padres, Chapman was clocked at 105.1 mph (169.1 km/h), according to PITCHf/x, the fastest pitch ever recorded in Major League Baseball. On July 19, 2016, Chapman matched his previous record of 105.1 mph with a ball to Baltimore's J. J. Hardy. That record was tied by Jordan Hicks on May 20, 2018.