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Ambulance and police were called to the south end of St Clair Beach about 4.30pm on Thursday after reports of a male in trouble in the water, police said.
A police spokeswoman said surf lifesavers were helping with the search.
A helicopter scoured the area including nearby Second Beach earlier, but had since been stood down.
The search would continue until dark and be reassessed on Friday morning, she said.
The section of road leading to the Second Beach access walkway was closed and LandSAR personnel were there for a time.
Water conditions at St Clair Beach were calm on Thursday, with a light breeze.