The suicide bomber targeted a checkpoint manned by Free Syrian Army rebels that was crowded with civilians early Friday in the village of Sousian, northwest of al-Bab.
Turkey-backed rebels on Thursday drove Daesh from al-Bab, the terror group's last significant stronghold in northwest Syria, along with two smaller neighboring towns of Qabasin and al-Bezah after weeks of street fighting.
Al Jazeera's Andrew Simmons, reporting from Gaziantep on the Syria-Turkey border, said the attack took place about 10km outside of al-Bab.
The medical charity Ambulanciers Sans Frontieres said 45 people, mostly civilians, had been killed and about 70 others were wounded.
"A large number of civilians were gathering around a checkpoint and suddenly a suicide car bomber drove in and detonated," Simmons said.
"There could be Free Syrian Army members counted in the casualty figures as well. This is really a warning shot from ISIL (Daesh) it would appear."
The car bomb hit outside a security office where civilians had gathered seeking permission to return to al-Bab.
Daesh said in a social media posting that it was behind the attack.
"There are still lots of cells inside Bab. It is very dangerous. Our search-and-clear operation is still under way," the rebel fighter said.
On Thursday, several Turkey-backed rebels were killed by a mine in al-Bab while clearing the town of unexploded ordnance after Daesh retreated, the group said.