Hamilton Public Health Services is advising residents and visitors to avoid swimming or wading near the shoreline and in shallow calm areas along the western end of Hamilton Harbour.
Blue-green algae has been found along the shoreline, with positive tests are both Bayfront and Pier 4 Parks.
Adverse health effects, such as headaches, abdominal pain and nausea, are mainly caused by drinking water that is contaminated with blue-green algae toxins. Skin contact with the toxins can cause irritation and itching.
Area beaches have been closed to swimming and pet owners are also urged to keep their dogs out of the water.