A heroic Scottish swimmer known as 'The Merman' rescued a Spaniard drowning on a tropical beach in Thailand.

Scott Siranat, a dual nationality Thai-Scottish conservationist, was out on patrol with marine officers when he spotted the Spanish tourist flailing in the sea off Koh Hong island in Krabi province on August 29.

Scott, 25, did not hesitate to plunge into the water and paddled his way toward the drowning man. He grabbed the gasping holidaymaker and dragged him back to safety toward the shore.

Speaking in the video, Scott said he was at sea with staff from the Than Bok Khorani National Park when he noticed a group of six tourists wading into the sea some 50 metres off the beach.

However, one of them appeared to be having trouble staying afloat.

The hero swimmer said: 'I swam over to help him, and he grasped my shoulder as if asking for help.

'I saw that his snorkel was filling up with water, so I took it off and pulled him back to the coast. It was good we saw him right away. Any later and he could have suffered a far worse fate.'

The Spanish tourist was treated by local medics and is now safe.

Experienced swimmer Scott earned the moniker 'The Merman' in April this year when he set a new record, becoming the youngest person in Asia to swim unassisted for 50 kilometres.

He swam across Krabi, Phang Nga, and Phuket provinces in southern Thailand in April to promote sea conservation.