Republic of San Marino
- Entered the war: 3 June 1915 (declared war on Austria-Hungary)
Republic of Panama
- Entered the war: 7 April 1917 (declared war on Germany)
- 10 December 1917 – declared war on Austria-Hungary
Republic of Cuba
- Entered the war: 7 April 1917 (declared war on Germany)
- 16 December 1917 – declared war on Austria-Hungary
Kingdom of Greece
- Entered the war: 27 June 1917 (declared war on the Central Powers)
Kingdom of Siam (Thailand)
- Entered the war: 22 July 1917 (declared war on Germany)
Republic of Liberia
- Entered the war: 4 August 1917 (declared war on Germany)
Republic of Brazil
- Entered the war: 26 October 1917 (declared war on Germany)
Republic of Guatemala
- Entered the war: 23 April 1918 (declared war on Germany)
Republic of Nicaragua
- Entered the war: 8 May 1918 (declared war on Germany and Austria-Hungary)
Republic of Costa Rica
- Entered the war: 23 May 1918 (declared war on Germany)
Republic of Haiti
- Entered the war: 12 July 1918 (declared war on Germany)
Republic of Honduras
- Entered the war: 19 July 1918 (declared war on Germany)