The vessel Smyrtos was sailing under a Cameroonian flag.
Royal Marine Commandos and National Crime Agency (NCA) officials boarded it in the early hours on Sunday, with support from Chinook helicopters and other aircraft, a frigate and a minehunter.
"This successful operation delivers yet another blow to Russia and reminds those fuelling Putin's war in Ukraine that we will not let them hide," Prime Minister Keir Starmer said in a post on X.
Video of the raid released by the government showed British commandos abseiling onto the vessel, while NCA officials were seen searching through documentation.
Lieutenant Colonel Tom Quinn told reporters that while there were some risks involved in the operation, it was a core part of the commandos' training and they had not faced any resistance.
"Once we boarded the vessel and moved to the bridge, the dialogue that we had with them was professional, safe, and indeed the crew enabled us to conduct the actions that we needed to complete to safely take control of that vessel and move it to an anchorage," he said.
Ukraine's President Volodymyr Zelenskiy wrote on X that he was grateful to Britain for the tanker's detention.
Kyiv has been lobbying its allies to toughen enforcement against the shadow fleet, which it says funds Russia's war machine.
"Europe urgently needs to take legislative steps to enable not only the detention of tankers and restrictions on oil shipments, but also the confiscation of the oil they carry. This will certainly help bring peace closer," he said on X.
The Russian embassy in London did not immediately respond to a request for comment.
The tanker will be detained and monitored off the south coast of England as investigations continue, a government statement said, adding that the operation was conducted in close coordination with the French.
Starmer, who is facing a potential leadership challenge and lost his defence secretary last week over a dispute about military spending, had in March permitted the British military to board and detain Russian ships, which Western governments say help Moscow export oil despite sanctions.
The prime minister's change of stance did not immediately have any impact on the number of Russian vessels under sanctions passing through UK waters, according to a Reuters analysis , which showed a similar number transited its waters before and after the announcement.
Until Sunday, Britain's involvement in efforts to stop the vessels had been limited to supporting French and US operations.
