Coastguards have spied what they believe is a group of about 70 pilot whales swimming in the Bristol Channel at Mumbles, near Swansea.
Pilot whales usually travel in family groups
The Swansea-based team made the first sighting just after 0830 BST on Sunday.
A lifeboat team from Port Talbot launched later in the morning found the pod of whales approximately three-and-a-half miles west of Mumbles.
The whales are said to be heading in a south-easterly direction along the channel towards Cardiff.
Nerys Sanders, coastguard watch assistant at Swansea Coastguards, said staff at the base near Mumbles lighthouse first saw about 60 or 70 of the dark blue-grey whales about half a mile from the shore.
"It was very impressive, I have never seen anything like it. It's not something you expect to see off the coast of south Wales."
She said her colleague, who had 20 years experience, had never seen whales so close before.
She added the animals were jumping in and out of the water.
The whales are not thought to be in any danger.
Pilot whales, which are actually dolphins by classification, can grow to over seven metres, weigh up to 3.5 tonnes and generally prefer deep waters.