Greenpeace International said activists from the Netherlands, Germany and Denmark had boarded Shell's Brent Alpha and Bravo platforms as part of a protest against plans by the company to leave parts of old structures in the North Sea.
The environmental group said climbers scaled the platforms, which are no longer operational and lie north-east of the Shetland Islands, and hung banners which read: "Shell, clean up your mess!" and "Stop Ocean Pollution".
Greenpeace confirmed on Tuesday that five activists had left the platforms after spending more than 24 hours on board.
It said that Shell's decommissioning plans will leave parts of four Brent oil platforms at sea with a total of around 640,000 cubic metres of oily water and 40,000 cubic metres of oily sediment containing more than 11,000 tonnes of oil.
Campaigner Christian Bissau added: "Our activists have made sure that Shell's refusal to clean up its own mess won't go unnoticed.
"This is a company that prides itself on being able to extract fossil fuels miles deep into the Earth's crust, and yet they say it's too hard to extract oil sludge from a few feet of concrete in their own rigs.
"This multi-billion-pound oil giant is just trying to skimp and save at the expense of our marine environment.
"If our governments are serious about protecting our seas, they should never allow oil companies like Shell to treat them as a waste dump."
Greenpeace International said that although a ban on dumping installations and platforms in the North East Atlantic ocean was agreed in 1998, Shell has requested an exemption from the UK Government.
It is calling for governments to protect the ocean and "not cave in to corporate pressure".
But a Shell spokesman said on Monday that the company had spent 10 years conducting in-depth research into decommissioning the Brent platforms and its recommendations were the result of more than 300 scientific and technical studies.