The men's 400 metres T12 event at the 2020 Summer Paralympics in Tokyo, took place between 31 August and 2 September 2021.

The T12 category is for athletes with visual impairment.

Athletes in this category will generally have some residual sight, the ability to recognise the shape of a hand at a distance of 2 metres and the ability to perceive clearly will be no more than 2/60.

T12 athletes commonly run with guides.

Prior to the competition, the existing records were as follows:[1] Heat 1 took place on 31 August 2021, at 21:17:[2] Heat 2 took place on 31 August 2021, at 21:25:[3] Heat 3 took place on 31 August 2021, at 21:33:[4] The final took place on 2 September 2021, at 9:42:[5]