The Army of the Duchy of Warsaw refers to the military forces of the Duchy of Warsaw. The Army was significantly based on the Polish Legions; it numbered about 30,000 and was expanded during wartime to almost 100,000. It was composed of infantry with a strong cavalry force supported by artillery. The Napoleonic customs and traditions resulted in some social tensions, but are generally credited with helpful modernization and useful reforms.

The Polish Legions were several Polish military units that served with the French Army in the Napoleonic era, mainly from 1797 to 1803, although some units continued to serve until 1815.