A local official said that the remote-controlled explosive device had ripped through the bus in Data Khel village near Miranshah, the main town in North Waziristan, early on Monday.
"Four people including a child and a woman were killed on the spot and another died later in Miranshah hospital," the official said on condition of anonymity.
The incident happened in the troubled North Waziristan region bordering Afghanistan, where violence continues weeks after battles between Pakistani soldiers and suspected pro-Taliban militants.
Separately, a truck coming from Afghanistan was hit by another roadside bomb in the Data Khel area but there were no casualties, the official said.
Also on Monday two tribesmen were killed and three paramilitary soldiers were wounded during a gun battle in a bazaar in Mir Ali town.
A Pakistani official said the clash erupted when troops tried to arrest the men for carrying assault rifles in public in defiance of a government ban imposed after last month's clashes.
Pakistan, an ally of America in its "war on terror", has deployed 80,000 troops along the border to hunt down militants who sought refuge there after the Taliban regime was ousted from Afghanistan in late 2001.