The number `` 42 '' was retired by all of Major League Baseball in 1997 because it was used by Jackie Robinson , who was the first black player in Major League Baseball . Players who were already wearing # 42 were allowed to keep using the number , but nobody else could use it . In 1997 , Major League Baseball , for the first time ever , made a Major League-wide retirement of a number , when # 42 could not be issued to any new players , having been retired in honor of Jackie Robinson , although all players who currently wore the number upon the mass retirement of # 42 , such as Mo Vaughn and Butch Huskey of the Red Sox and Mets , were allowed to keep it under a grandfather clause if they were wearing the number in honor of Jackie Robinson .