| Unit Name: | Japanese Thirty First Army |
| Dates: | February 18, 1944 - August 15, 1945 |
| Country: | Empire of Japan |
| Branch: | Imperial Japanese Army |
| Type: | Infantry |
| Role: | Corps |
| Garrison: | Truk |
| Battles: | Operation Hailstone Battle of Saipan Battle of Guam |
The was an army of the Imperial Japanese Army during World War II.
The Japanese 31st Army was formed on February 18, 1944 under the Imperial General Headquarters as a garrison force to contest landings by Allied forces in the Japanese South Seas Mandate island-by-island, and to inflict such losses in a war of attrition that it would deter an American invasion of the Japanese home islands. The South Seas Mandate was divided into three sections (Northern Mariana Islands, southern Mariana Islands, and Truk). The 80,000 man Japanese 31st Army was initially headquartered on Truk.
After Operation Hailstone, the Japanese garrison on Truk was isolated as American forces continued their advance towards Japan by invading other Pacific islands. Cut off, the Japanese forces on Truk and other central Pacific islands ran low on food and faced starvation before Japan surrendered in August 1945. (Stewart, 1986)
The garrisons in the Marianas were largely annihilated at the Battle of Saipan and Battle of Guam.
| Name | From | To | ||
| 1 | 25 February 1944 | 11 August 1944 | ||
| 2 | 11 August 1944 | 22 August 1944 | ||
| 3 | 22 August 1944 | 1 September 1945 |
| Name | From | To | ||
| 1 | 25 February 1944 | 6 July 1944 | ||
| 2 | 14 July 1944 | 11 August 1944 |