Reaves began playing hockey at the age of five and football at the age of eight. As a youth, Reaves played in the 2000 Quebec International Pee-Wee Hockey Tournament with the Winnipeg South Monarchs minor ice hockey team. Although he played both football and hockey growing up, after tearing his PCL during a hockey tournament, he was forced to forfeit football as a sport. His torn PCL made him unable to compete in the football season prior to his Western Hockey League draft year. Reaves was eventually drafted 36th overall by the Brandon Wheat Kings in the 2002 WHL bantam draft. Despite his draft selection, Reaves continued to play for the St. John's-Ravenscourt School hockey team where he led them to the 2004 McDonald's Provincial High School Hockey Championship. Reaves joined the Wheat Kings for his rookie campaign during the 2004–05 season. Prior to the 2005–06 season, Reaves was named an alternate captain alongside Mark Derlago, Riley Day, and Derek LeBlanc.
Professional
St. Louis Blues
Reaves was drafted by the St. Louis Blues 156th overall in the 2005 NHL Entry Draft. On May 15, 2007, Reaves signed a three year entry level contract with the St. Louis Blues. At the conclusion of his major junior season, Reaves attended the Blues 2007 training camp and was reassigned to their American Hockey League affiliate, the Peoria Rivermen. While playing with the Rivermen during the 2007–08 season, he suffered an injury in a game against the Rockford IceHogs and was reassigned to the Blues ECHL affiliate, the Alaska Aces. Reaves was recalled from the Peoria Rivermen by the Blues on October 10, 2010, after Cam Janssen suffered a concussion from a hit to the head in the Blues season opener on October 9. Reaves made his NHL debut on October 11, 2010, where he served 15 penalty minutes in the Blues' 5–1 win over the Anaheim Ducks. He was returned to Peoria after two games with the Blues as Vladimír Sobotka returned from the Injured reserve list. He later scored his first NHL goal on January 12, 2011, again against the Ducks, on goaltender Jonas Hiller. Reaves' 2015–16 season was cut short due to a knee injury and he returned to the Blues' lineup weighing 225 instead of his usual 230. During the offseason, he trained with Blues assistant coach/video coach Sean Ferrell.
Pittsburgh Penguins
On June 23, 2017, after seven seasons with the Blues, Reaves was traded to the Pittsburgh Penguins along with a second-round pick in the 2017 NHL Entry Draft in exchange for Oskar Sundqvist and the Penguins' first-round pick in the 2017 draft. In the 2017–18 season, Reaves added muscle to the Penguins lower lines, providing 4 goals and 8 points in 58 games.
On February 23, 2018, Reaves was involved in a trade with the Vegas Golden Knights. The Penguins received Tobias Lindberg from the Golden Knights in exchange for Reaves as part of a three-way trade also involving the Ottawa Senators. Reaves scored the series-winning goal over the Winnipeg Jets that qualified the Golden Knights for the 2018 Stanley Cup Finals. He also scored in Game 1 of the Stanley Cup Finals in a 6–4 Golden Knights victory, though Washington would come back and win the next four games. On July 1, 2018, Reaves signed a two-year contract with the Golden Knights. He recorded a career-high nine goals and 20 points the subsequent season. On June 15, 2020, the Golden Knights signed Reaves to a two-year, $3.5 million contract extension.
Playing style
Reaves has been described in the NHL as an enforcer.
