A Hoops fan climbed the iconic building to change the flag.
The farce was short-lived as the flag was quickly returned to the long-standing Saltire.
Glasgow city council called the waterfall “inappropriate” and launched an investigation.
Conservative Councilor Thomas Kerr tweeted, “Concerned to see a Celtic flag fly from city rooms this morning.
“Our municipal buildings are there for all Glaswegians, regardless of which football team you choose to support.
“I spoke to Lord Provost who assured me that this matter was under investigation.”
A Glasgow City Council spokesperson said, “This is inappropriate. This should not have happened and we are investigating. “