The Blue Army, or Haller's Army, was a Polish military contingent created in France during the latter stages of World War I. The name came from the French-issued blue military uniforms worn by the soldiers. The symbolic term used to describe the troops was subsequently adopted by General Józef Haller von Hallenburg himself to represent all newly organized Polish Legions fighting in western Europe.

Józef Haller von Hallenburg was a lieutenant general of the Polish Army, a legionary in the Polish Legions, harcmistrz, the president of the Polish Scouting and Guiding Association (ZHP), and a political and social activist. He was the cousin of Stanisław Haller.