The mine exploded after the vehicle in which the soldiers were travelling at Kaiga Kindji, close to the border of Chad and Niger drove over it.
About 12 other soldiers were wounded.
The incident is the latest in the series of attacks on Chadian soldiers by Boko Haram extremists.
The Nigerian-based Boko Haram wants to create a breakaway Islamic state in the region and once occupied an area the size of Belgium.
But a regional offensive led by Nigeria, Chad, Cameroon and Niger – the four countries most affected by Boko Haram – has chased it out of much of that territory.