| Unit Name: | Eighteenth Area Army |
| Dates: | January 4, 1943 - August 15, 1945 |
| Country: | Empire of Japan |
| Branch: | Imperial Japanese Army |
| Type: | Infantry |
| Role: | Field Army |
| Garrison: | Bangkok |
| Nickname: | 義 (Gi = “righteous”) |
The was a field army of the Imperial Japanese Army during World War II.
The 18th Area Army was originally formed on January 4, 1943, as the . It was renamed the Thirty-ninth Army on December 14, 1944, and became the 18th Area Army on July 7, 1945, shortly before the end of the Pacific War.[1]
The 18th Area Army was under the control of the Southern Expeditionary Army Group as a military reserve and garrison force, ostensibly to help defend the Empire of Japan's nominal ally, the Kingdom of Thailand against possible invasion by the Allies, but in more practical terms, it was present to ensure that Thailand remained an ally to Japan. It was headquartered in Bangkok.
The 18th Area Army was demobilized in Bangkok at the surrender of Japan on August 15, 1945, without having seen combat.
| Name | From | To | Notes | |
| 1 | 4 January 1943 | 20 December 1944 | Siam Garrison Army | |
| x | 20 December 1944 | 14 July 1945 | Japanese 39th Army | |
| x | 14 July 1945 | 15 August 1945 | 18th Area Army | |
| Name | From | To | Notes | |
| 1 | 4 January 1943 | 21 January 1943 | Siam Garrison Army | |
| 2 | 21 January 1943 | 22 November 1944 | Siam Garrison Army | |
| 3 | 22 November 1944 | 20 December 1944 | Siam Garrison Army | |
| 1 | 20 December 1944 | 9 July 1945 | Japanese 39th Army | |
| 2 | 9 July 1945 | 14 July 1945 | Japanese 39th Army | |
| 1 | 14 July 1945 | 15 August 1945 | Japanese 18th Area Army | |