"The bomb was placed in a cart by the roadside. It was detonated as the bus arrived," said Asif Waziri, of the Balkh police department in Mazar-i-Sharif, on Tuesday.
Waziri told AFP news agency that Tuesday's blast happened around 7:00 am (0230 GMT) near Sayed Abad Square in the city.
He said six people were injured in the blast.
Further details were not immediately available, and there has been no claim of responsibility.
At least 19 people were killed and 24 others wounded earlier this month by a blast at a madrassa in Aybak, southeast of Mazar-i-Sharif.