The Portuguese Expeditionary Corps was the main military force from Portugal that fought in the Western Front, during World War I. Portuguese neutrality ended in 1916 after the Portuguese seizure of German merchant ships resulted in the German Empire declaring war on Portugal. The expeditionary force was raised soon after and included around 55,000 soldiers.

Expeditionary warfare is a military invasion of a foreign territory, especially away from established bases. Expeditionary forces were in part the antecedent of the modern concept of rapid deployment forces. Traditionally, expeditionary forces were essentially self-sustaining with an organic logistics capability and with a full array of supporting arms.