The attack happened Sunday in Balochistan province's Dringarh area as the buses were carrying the pilgrims from Quetta to Iran, The News International reported.
One bus was destroyed in the attack while the two others were damaged, said Tufail Baloch, deputy commissioner of the Mastung district.
Nineteen people, including four women, were killed in the attack, while at least 25 were injured, CNN reported.
Baloch said investigators found evidence that the bomb was detonated remotely.
It was not immediately clear who was responsible for the attack.