Pakistani military sources said the improvised explosive device (IED) was planted on roadside at Zatrai village in the tribal region bordering Afghanistan.
Two soldiers died on the spot while three others succumbed to their injuries at a hospital. Another seriously injured soldier is battling for life, the sources added.
South Waziristan, a site of frequent US drone strikes, is one of seven semi-autonomous tribal regions in Pakistan. These areas are a major hub of the Taliban and al-Qaeda-linked militant groups.