The 22lb (10kg) pot hit the 60-year-old at the wheel of the single decker on the M80 in Cumbernauld.
The pot smashed the steering column and the driver immediately lost all steering control, but he was able to slow the bus and bring it to a stop at the side of the motorway.
The tub is thought to have been stolen from a nearby building site.
Chief Inspector Glen Bland said: "It is no understatement to say it is only sheer luck that stopped this incident being much worse.
"The driver counts himself lucky he was not severely injured or killed as a result of this senseless act on one of Scotland's busiest roads.
"It's to the driver's credit he managed to avoid other vehicles and stop the bus at the edge of motorway."
The incident happened at Junction 6 northbound about 9.50pm on Saturday.
The driver suffered minor injuries but was badly shaken, police said.
No passengers were on the bus.
Officers are viewing CCTV footage as they hunt for the culprit.
The large tub may have been stolen from a nearby building site close to Our Ladies High School in Dowanfield Road, Seafar.
Chief Inspector Bland added: "It is vital we trace those responsible for carrying out such a dangerous act.
"I am appealing to drivers who were on the M80 and saw anyone on the overhead bridge near Junction 6 to contact me.
"Also, anybody who saw anyone hanging around the Dowanfield Road area or someone carrying a large white plastic tub that night should get in touch."