The popular beaches at Silver Strand and Salthill and Blackrock diving board are all affected by a prohibition placed by Galway City Council.
It has also advised against swimming at Grattan beach in lower Salthill.
Very high levels of an intestinal enterococci bacteria were detected during routine testing and the bacteria may cause illness.
Galway City Council says it is investigating the cause, and the prohibition is in place “until further notice”.
It says that further testing is taking place, but it may be Saturday (June 1) before results are available.