The Ford lorry was driving anticlockwise on the M25 just past junction eight when he heard a loud bang on the windscreen as he drove beneath a footbridge at around 10:30am on March 2.
His windscreen cracked but he was able to keep control of the vehicle and remain uninjured.
Surrey Police is now appealing for information so they can track down those responsible for the "mindless act".
Acting detective inspector Tony Giles said: "It is extremely dangerous to throw objects from bridges into the paths of on-coming traffic.
"On this occasion the driver was able to keep control of the lorry but a distraction of this magnitude could have easily caused him to lose control and collide with another vehicle on the busy motorway."
The driver saw two people standing on the footbridge prior to the incident – and officers are keen to trace them.
DI Giles said: "The consequences could have been much worse and, as a result of one mindless act, the culprits could have been facing charges of manslaughter or even murder."
Anyone with information should call Surrey Police on 0845 125 2222 quoting reference RB/10/1937.