The infantry soldier in the Rifles regiment died on Tuesday in the Brecon Beacons area of Wales, the Ministry Of Defence said.
An Army spokesperson said: "We can confirm that a soldier from ITC Catterick died on 19 July while conducting pre-course training for the Platoon Sergeants’ Battle Course in Brecon."
It is the fourth such death in that area in three years.
In 2013 three soldiers died while taking part in an SAS training exercise in the Brecon Beacons during 29.5C heat - one of the hottest days of that year.
Lance Corporal Craig Roberts died during the march and Lance Corporal Edward Maher and Corporal James Dunsby collapsed and died later.
Ten other soldiers also suffered heat illness during the expedition.
A coroner ruled that neglect played a part in their deaths.
Temperatures on Tuesday peaked at just above 30C (86F) in Brecon, the Met Office said.
Soldier dies in training exercise in Brecon Beacons on hottest day of year so far
20 July 2016
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A British soldier has died during a training exercise on the hottest day of the year so far.
