Kuniaki Koiso was arrested at the end of the war and was tried by the International Military Tribunal for the Far East (IMTFE) for war crimes as a Class-A war criminal.
He was found guilty on count 1 (conspiracy to wage wars of aggression against one or more countries, between 1 January 1928 and 2 September 1945) and counts 27, 29, 31 and 32 (for having waged wars of aggression) and count 55 (for not fulfilling his duties as a military commander by disregarding his duty to take adequate steps to prevent violations of the laws and customs of war).
In addition, the Tribunal concluded that due to his high rank in the Japanese military hierarchy, Kuniaki Koiso had played a decisive role in starting the wars against China and the allies. Furthermore, despite the fact that Kuniaki Koiso was not the instigator of the war crimes committed by the Japanese Army, he took no measures to prevent them or to punish the perpetrators when, as Prime Minister, it was fully in his power to do so.
On 12 November 1948, the IMTFE sentenced him to life imprisonment.