The soldiers were reportedly carrying out routine search operations in Makeen Tehsil of South Waziristan district when the explosion took place.
While there was no immediate claim of responsibility for the bomb attack, the military has been battling Taliban and Al Qaeda linked militants in the region for years.
Violence across the country has declined in recent years following a series of military operations along the border with Afghanistan, but militant groups are still able to carry out deadly attacks.
There have been a number of fatal assaults in recent months in the northwest, as well as in Balochistan.
The army launched an operation in mid-2014 to wipe out militant bases in the area and end the near decade-long insurgency that has cost thousands of lives.
The operation was intensified after the Taliban massacred more than 150 people, the majority of them children, in the attack on the Army Public School (APS) in Peshawar in December 2014.
In 2016, the army said it has cleared the last militant stronghold in the country’s northwest after a three-month-long operation.