Astonished motorists saw the child standing on the van's back step and hanging on to a door handle as the vehicle travelled at up to 80 kilometres an hour through hill roads near Delliehausen, in northern Germany.
Police gave chase and pulled the van over about 18 kilometres later, just as the driver's boss reached him by cellphone and ordered him to stop.
The child, who was not named by police, was cheerful and unfazed as she hopped off the vehicle, police said.
Police said the grandfather, 61, had been playing with her before driving away and had not noticed her climbing on the step at the back of the vehicle.
"He never suspected she was there," a spokeswoman said.