The number 21 in baseball is most famously associated with Roberto Clemente, a legendary Puerto Rican professional baseball player who played for the Pittsburgh Pirates. Clemente wore number 21 for the majority of his career and is celebrated not only for his outstanding achievements on the field but also for his humanitarian efforts off the field. His number 21 has been retired by the Pittsburgh Pirates in his honor, and MLB commemorates Roberto Clemente Day every year with players often wearing number 21 to honor his legacy.