Reginald Martinez Jackson (born May 18, 1946) is an American former professional baseball right fielder who played 21 seasons in Major League Baseball (MLB) for the Kansas City / Oakland Athletics, Baltimore Orioles, New York Yankees, and California Angels. He led his teams to first place eleven times over his 21-year baseball career and had only two losing seasons. He was inducted into the National Baseball Hall of Fame in 1993.

He was nicknamed "Mr. October" for his clutch hitting in the postseason with the Athletics and the Yankees. He helped Oakland win five consecutive American League West divisional titles, three straight American League pennants and three consecutive World Series titles from 1972 to 1974. He helped New York win four American League East divisional pennants, three American League pennants and back to back World Series titles, in 1977 and 1978. He also helped the California Angels win two AL West divisional titles in 1982 and 1986. He hit three consecutive home runs at Yankee Stadium in the clinching game six of the 1977 World Series.

He hit 563 career home runs and was an American League (AL) All-Star for 14 seasons. He won two Silver Slugger Awards, the AL Most Valuable Player (MVP) Award in 1973, two World Series MVP Awards and the Babe Ruth Award in 1977. The Yankees retired his uniform number in 1993, and the Athletics retired it in 2004. He currently serves as a special advisor to the Houston Astros, and a sixth championship associated with him came with Houston's win in the 2022 World Series.

He was the de facto spokesperson for the Upper Deck Company during the early 1990s, appearing in numerous advertisements, appearances, and participating in the company's Heroes of Baseball exhibition games.

He has endured three fires to personal property, including a June 20, 1976, fire at his home in Oakland that destroyed his 1973 MVP award, World Series trophies and All-Star rings. The same home was again burned down during the Oakland firestorm of 1991, which destroyed more baseball memorabilia in addition to other valuable collections. In 1988, a warehouse holding several of Jackson's collectible cars was damaged in a fire, with several of the cars, valued at $3.2 million (~$8 million in 2022 terms) ruined.

He co-authored a book in 2010, Sixty-Feet Six-Inches, with fellow Hall of Famer Bob Gibson. The book, whose title refers to the distance between the pitcher's mound and home plate, details their careers and approach to the game.