Charles met with volunteers at Scrabster Beach in Caithness on the northern tip of the United Kingdom on Thursday.
He will take part in several engagements in the town of Thurso during his visit to Scotland where he is known as the Duke of Rothesay.
Wearing a kilt in Rothesay tartan, Charles met with Dorcas and Allan Sinclair, founders of the Caithness Beach Clean Group.
They showed the prince some of the objects they have collected from the more than 3,500 beach cleans carried out since the group was founded in 2019.
The objects included a coaxial cable which Ms Sinclair told the Prince they were holding on to in case they could find someone to use it.
Speaking to the PA news agency ahead of the visit, Ms Sinclair, 64, said the group had picked up more than 31 tonnes of plastic since they started.
"We find horrible things like syringes, needles and even a gas mask," she said.
"Recently we found a giant buoy, one of the cleaners got a tiny leather child's shoe that is absolutely ancient.
"There's also lots of pants, would you believe? So many people find men's pants - only men's, never women's - and we talk about having a 'pantometer' because there's a running joke about who finds the most pants."
Ms Sinclair and her husband, who live four miles outside Thurso, go to six to eight beaches on the north coast of Scotland a week.
She said: "The beaches here are now so good we've got to travel because there's not as much to clean
"We're also really trying to include children in the beach cleans because it's their world.
"If a child goes out with their parents, I send them a certificate and a badge, we run competitions for them and we've got them designing things like dog poo posters."
On Charles's visit, she said: "He's such an environmentalist himself, we're really, really pleased he's recognising what we're doing here."
The heir to the throne later visited DS McGregor & Partners veterinary surgery in Thurso.
The practice treats both animals, including household pets, equine and farm animals, and featured in The Highland Vet TV series.
He was welcomed by senior vet and director Guy Gordon, who introduced him to his team, including Katie Reiss, 22, who only started work on Monday.
Ms Reiss said: "It's an unorthodox start to work! We spoke about my training at Edinburgh University and chatted about how the vets have been really helpful integrating me into work.
"He (Charles) said to stick at it and not lose hope because I have wanted to be a vet since I was a wee kid."
Charles also visited the newly-restored House of the Northern Gate in Thurso and toured the building and gardens.
Queen Elizabeth The Queen Mother stayed at the house in 1953 after the death of her husband, King George VI.
It was here, at the most northerly point of mainland Britain, that she spotted the tower of the recently vacated Barrogill Castle.
A deal was struck to buy the dilapidated building and the Queen Mother renamed it the Castle of Mey, its original name.