Several of the animals, which are known to prefer colder waters, were seen from one of the ferries which plies the estuary of the Scottish river on Saturday.
Another, Maureen Campbell, wrote on Facebook: “Best ferry journey ever. Amazing!!”
Pictures circulated on social media after the sightings between Dunoon in Argyll and Bute, and Gourock, Renfrewshire.
The pod, believed to be made up of five adults and a youngster, were also spotted in the stretch of water between Innellan and Wemyss Bay.
One of them was photographed jumping out of the water.
— Oor Lorna (@OorLorna) April 21, 2018
Corinne Gordon, out of hours coordinator for British Divers Marine Life Rescue, which helps stranded animals in UK waters, said her organisation had been monitoring the pod all day and they were safe.
“There are out in deep waters and are safe,” she said. “There was a pod of six – one bull, one calf and four females – and about 8.45pm they were heading west towards Greenock, in a deep shipping lane and away from danger.
“At one point they were heading east and got past Dumbarton and as far as up by Erskine Bridge, following food – shoals of fish and seals. There are plenty of seals in the Clyde at the moment.
“These are a pod that generally reside in Orkney and they’ve come down. It could be the weather or it could be food.
A couple of grainy pics of the Killer Whales in the Clyde today taken from my folks' flat. The belter with the jumping whale was taken by Keith Hodgins. pic.twitter.com/McW2MngFci
— Christian (@cbarry40) April 21, 2018
“We had one come into the Clyde couple of years ago. They are not strangers there but it’s not common.
“We are expecting them to go back out to sea and then head north.”
Orcas, as they are also known, are regularly seen in waters off Shetland and Orkney during the summer months.
They can travel of groups of up to 50.
The feeding grounds for the majority of north Atlantic orcas are off the coasts of Norway and Iceland.