The Boston Red Sox are wearing 21 on their jerseys as part of Major League Baseball’s tribute to Hall of Famer Roberto Clemente, using his number to honor his legacy as a player and humanitarian, especially around Roberto Clemente Day on September 15.

Quick Scoop: What’s up with 21?

When you see the Red Sox with 21 on their jerseys, it’s not a new permanent number change but a themed tribute tied to league-wide Clemente celebrations. MLB allows players, coaches, and especially those of Puerto Rican descent, plus Roberto Clemente Award nominees, to wear 21 or a 21 patch on Clemente Day.

The Clemente connection

  • Number 21 is famous because it was worn by Roberto Clemente, a Puerto Rican star outfielder for the Pittsburgh Pirates, 12-time All-Star and two-time World Series champion.
  • MLB created Roberto Clemente Day to honor his on‑field excellence and his humanitarian work, which cost him his life in a 1972 plane crash while delivering earthquake relief supplies.
  • On that day, teams like the Red Sox often have multiple players and staff wear 21 or a visible 21 patch.

Specifically for the Red Sox

  • The Red Sox have had several Puerto Rican players and staff in recent years, including Alex Cora and others who chose to wear 21 with pride on Clemente Day.
  • In those games, you might see multiple Red Sox jerseys showing 21 at once, making it look like a team‑wide number, but it’s really a special‑occasion tribute rather than a standard uniform change.

Why it’s trending now

Forum and fan discussions often spike whenever an MLB date rolls around where many players across the league wear 21 at the same time. That’s why you’ll see questions like “why do the Red Sox have 21 on their jerseys” pop up on social media and fan boards around those tribute dates.

In short: it’s a visual salute to Roberto Clemente and the values he represents—excellence, community service, and humanitarianism—not a mystery roster move or a new permanent Red Sox number choice.

TL;DR: The Red Sox wear 21 on their jerseys during league‑wide tributes to Roberto Clemente—especially on Roberto Clemente Day—as a special honor to his legacy, not as a permanent number change.

Information gathered from public forums or data available on the internet and portrayed here.