The Scottish Environment Protection Agency (SEPA) said it had received a number of calls about the foam, and launched what they called a "pollution incident".
They later determined the foam to be the result of an illegal discharge of detergent into the sewer network.
The pale bubbles stretched from the Broomielaw in the city centre down towards the Clyde Arc, also known as the Squinty Bridge.
They were thought to originate from an overflow pipe near the Tradeston Bridge, known locally as the Squiggly Bridge.
A Sepa spokesman said: "In liaising with Scottish Water, the source of pollution on the River Clyde is now believed to be as a result of an illegal discharge of detergent to the sewer network.
"Scottish Water is currently in the process of carrying out an investigation to trace the origin of the discharge and Sepa will continue to monitor the situation while carrying out analysis of the samples taken in order to ascertain the nature of the material."
A Scottish Water spokesman said: "Scottish Water has been working with the Scottish Environment Protection Agency (Sepa) and investigating an incident on the River Clyde in Glasgow.
Scottish Water has been carrying out some work to clean a pedestrian walkway near the outfall where the incident occurred.
Another discharge of foam was also reported in the Thornliebank area in East Renfrewshire, which Sepa said they were also investigating.
The spokesman said: "Sepa is aware of another discharge of foam on the Auldhouse Burn and White Cart Water near Thornliebank and Sepa officers have also carried out a separate investigation to assess the potential impact on the local environment."