This is a list of countries that participated in World War I, sorted by alphabetical order.

Allies

• Belgium
• Brazil
• China
• Costa Rica
• Cuba
• France
• Greece
• Guatemala
• Haiti
• Honduras
• Italy
• Japan
• Liberia
• Montenegro
• Nicaragua
• Panama
• Portugal
• Romania
• Russian Empire
• San Marino
• Serbia
• Siam (now Thailand)
• Great Britain
• United States of America

German coalition

• Austro-Hungary
• Bulgaria
• German Empire
• Ottoman Empire

Battlefields

• Africa
• Arabia
• China
• Europe
• Atlantic Ocean
• Pacific Ocean

Other notes

The British Empire was the common term for Great Britain at this time. The term refers to the colonies it controlled, and the British Dominions - former colonies which were self-governing in most respects but whose foreign policy was controlled by London; Canada, Australia, New Zealand and South Africa.