Mark John Douglas Messier OC (/ˈmɛsieɪ/; born January 18, 1961) is a Canadian former professional ice hockey forward. His playing career in the National Hockey League (NHL) lasted 25 seasons (1979–2004) with the Edmonton Oilers, New York Rangers, and Vancouver Canucks. He also played professionally with the World Hockey Association (WHA)'s Indianapolis Racers and Cincinnati Stingers. He also played a short four-game stint in the original Central Hockey League (CHL) with the Houston Apollos in 1979. He was the last WHA player to be active in professional ice hockey, and the last active player in any of the major North American professional sports leagues to have played in the 1970s. After his playing career, he was special assistant to the president and general manager of the Rangers.

Messier is considered one of the greatest ice hockey players of all time. He is second on the all-time list for playoff points (295) and third for regular season games played (1756) and regular season points (1887). He is a six-time Stanley Cup champion—five with the Oilers and one with the Rangers—and is the only player to captain two teams to Stanley Cup championships. His playoff leadership during his first stint with the Rangers, which ended a 54-year Stanley Cup drought in 1994, earned him the nickname "The Messiah", a play on his name. He was also known, throughout his career, as "The Moose" for his aggression and strength. He twice won the Hart Memorial Trophy as the league's most valuable player, in 1990 and 1992, and in 1984 won the Conn Smythe Trophy as the most valuable player during the playoffs. He was selected for the NHL All-Star Game sixteen times. In 2007, he was inducted into the Hockey Hall of Fame, in his first year of eligibility. In 2017, Messier was named one of the "100 Greatest NHL Players" in history.

On June 30, 2017, he was named an Officer of the Order of Canada by Governor General David Johnston for "contributions to hockey as an outstanding player and captain, and for his leadership in encouraging children to take up the sport."

Messier was born in St. Albert, Alberta, the son of Mary-Jean (Dea) and Doug Messier. He was the second son and third child of four; his siblings are Paul, Mary-Kay, and Jennifer. The Messier family moved to Portland, Oregon when Mark was young, where Doug played for the Portland Buckaroos of the minor pro Western Hockey League. The family returned to St. Albert in 1969 after Doug retired from hockey. Messier attended St. Francis Xavier High School in Edmonton as he played junior hockey where Doug was his coach and mentor for his early years.

Messier's brother Paul was drafted by the Colorado Rockies 41st overall in the 1978 NHL Amateur Draft, but he only played nine games with the club in 1978–79 before embarking on a long career in the German Eishockey-Bundesliga. Paul helps manage a hotel that Messier owns in Harbour Island, Bahamas. Messier's cousins Mitch and Joby also skated for NHL clubs. Joby was briefly Mark's teammate on the Rangers.

In 1976, Messier tried out for the junior Spruce Grove Mets of the Alberta Junior Hockey League (AJHL), where his father, Doug Messier, was coaching. As Messier was only 15 (the age limit was 20), Doug did not expect him to make the team, though he was surprised and was added to the roster. Messier recorded 66 points in 57 games with the Mets in the 1976–77 season. The team, which relocated to St. Albert the following season and was renamed the St. Albert Saints, named Messier captain for 1977–78, and he scored 74 points in 54 games. After the season he joined the Portland Winterhawks of the major junior Western Hockey League (WHL) for the playoffs, appearing in 7 games and scoring 5 points.

Before the start of the 1978–79 season, Messier was looking for alternatives to another season with the Saints, as he felt he was too good for the AJHL. He was not interested in playing in the WHL, so he initially tried out for the Canadian Olympic team, which was preparing for the 1980 Winter Olympics. At the same time, Doug contacted his former junior teammate Pat Stapleton, who was coaching of the Indianapolis Racers of the World Hockey Association (WHA), who needed someone to replace another young player they had just traded, Wayne Gretzky. Doug called him and got Messier a contract to play hockey in Indianapolis for $30,000. However Messier only took a 5-game amateur tryout, as it would allow him to return to junior hockey if need be.

Messier played 13 games with the Saints to start the season, then joined the Racers on November 5 for his professional debut against the Winnipeg Jets. He played four games with the Racers, followed by a further two more with the Saints, before his final game with Indianapolis on November 28. Offered a longer contract, Messier held off on signing it, which proved fortuitous as the team folded on December 15; his only cheque from them bounced. Returning to the Saints, Messier played his final two games of junior hockey for them before he was signed by the Cincinnati Stingers, also of the WHA. Messier signed a contract for $35,000 to play the rest of the season with the Stingers. He scored his first professional goal on March 20, 1979, against Pat Riggin of the Birmingham Bulls. Messier played 47 games for the Stingers, tallying one goal and 10 assists.