Nolan James Arenado is an American professional baseball third baseman for the St. Louis Cardinals of Major League Baseball (MLB). Arenado is widely recognized as one of the best defensive third basemen of all time. He is the only infielder to win the Rawlings Gold Glove Award in each of his first ten MLB seasons. He made his MLB debut with the Colorado Rockies in 2013 and was traded to the Cardinals before the 2021 season.

A third baseman, abbreviated 3B, is the player in baseball or softball whose responsibility is to defend the area nearest to third base — the third of four bases a baserunner must touch in succession to score a run. In the scoring system used to record defensive plays, the third baseman is assigned the number 5.