A man is in a serious condition after being bitten by a shark at a beach in South Australia.

Emergency services were called to Port Noarlunga Jetty, south of Adelaide, around 9.45am local time (10.15am AEST) following reports of a shark attack.

The victim, a man in his 40s, suffered a significant injury to his thigh. He was treated at the scene before being transported to Flinders Medical Centre in a serious condition.

Swimmers were immediately cleared from the water, and police have urged the public to avoid the area while the investigation continues.