First Legion may refer to:
- The First Legion (1951 film) U.S. drama film
- Legio I (Ancient Rome), see List of Roman legions
- 1st Legion Tercio "Great Captain Gonzalo Fernández de Córdoba", an infantry regiment in the Spanish Legion
- 1st Legions Infantry Division (Poland), an infantry division in the Polish army
- 1st Brigade, Polish Legions, an infantry brigade in the Austro-Hungarian army