Shark sighting in Port Phillip Bay

'Shark' spotted off Chelsea beach. Photo: Facebook - Silversix Ben

A suspected shark sighting just a few metres off a bayside beach has swimmers on alert.

Witnesses say the three metre shark was spotted within three or four metres of the shoreline at Chelsea beach yesterday evening.

A video of what appears to be a fin protruding from the water has gone viral on social media.

The shark is believed to be a bronze whaler and witnesses say it was hunting salmon.

A local resident told Fairfax Radio he is convinced the sighting is legitimate.

"It was a shark definitely," he said.

"It was within three or four metres of the shore.

"I've lived by the beach all my life and it's the first shark I've seen."