Two men were arrested and charged following a raid on the property this week.
Officers seized 748 plants, worth £448,800, and sophisticated growing equipment.
The Main Street building had lain empty after the Royal Bank of Scotland branch closed in 2018.
Staff in nearby businesses were stunned to hear of the discovery.
One shop worker said: "As far as I knew the building was boarded up and empty. I didn't know a thing about this."
A colleague added: "Who would have thought such an operation would be running around the corner from the police station?"
Hung Van Ho, 20, and Hoang Nguyen, 18, appeared in private at Hamilton Sheriff Court yesterday.
They were accused of producing cannabis and bypassing the electricity meter at the former bank building.
Solicitors Diarmid Bruce and Gregor Jarrott made no plea on their behalf and the men were committed for further enquiries. Sheriff Ray Small remanded them in custody.
A Police Scotland spokesman said: "Police in Lanarkshire attended at an address in Main Street, Bellshill, on Tuesday, January 4, and found a cannabis cultivation within. Around 748 plants were seized, worth an estimated £448,800. Two men were arrested in connection with this investigation and a report was submitted to the procurator fiscal."
Police advise that the cultivations are not only illegal but present a major fire risk to to the properties involved. This is because overloaded electrical circuits with cheap wiring and lights present a a highly flammable set-up in a ‘greenhouse’ style environment.