The incident occurred around 1.20pm when the truck, driven by a 61-year-old man from the canton of Bern, travelled from Oberentfelden toward Suhr, Aargau cantonal police said.
The truck struck the cyclist near a pedestrian crossing, police said.
The victim, a resident of the region, fell under the vehicle, which the truck driver immediately stopped.
Rescue workers arrived on the scene to care for the schoolgirl but she succumbed from her injuries at the accident site, police said.
Officers provisionally took away the truck driver’s driving permit while a criminal investigation conducted by the local prosecutor’s office into the accident continues.
Aargau police are looking for witnesses who may have seen how the accident occurred.
In particular, police said they were looking for a middle-aged couple, aged around 50, who were on the opposite side of the road when the collision happened.
These people were no longer on the site when officers arrived, police said.