Ten Greenpeace activists were cleared of aggravated trespass on Friday after they prevented a tanker carrying Russian diesel from unloading on the River Thames near London.
The environmental campaigners occupied a jetty in Grays, eastern Essex, in May, stopping the delivery of 33,000 tonnes of fuel from a Greek-flagged vessel.
They claimed they were stopping the funding of Russian state terrorism in Ukraine but were charged with trespass and obstructing a "lawful activity, namely fuel distribution".
Judge Christopher Williams cleared all 10 after a trial at Chelmsford Magistrates Court, northeast of the British capital, prompting cheers and applause from the defendants.
"I take the view it's more than likely the Russian war could be described as terrorism," he said.
Greenpeace UK claimed in a statement that it was the first time a British court had agreed with the argument that the Kremlin's invasion of Ukraine was "terrorism".
The defendants produced several witnesses to testify that revenues from oil and gas exports were being used by Russia to fund the conflict.
One of the protesters who was cleared called the verdict "seismic" and said it vindicated their actions in full.
"The judge has recognised that we are just ordinary people whose conscience compelled us to stop the oil funding Putin's campaign of state terrorism in Ukraine," said Zoe Pontida, a 32-year-old teacher from Oxford.
"With every new town flattened by bombs and every new war crime being uncovered, the reason for stopping the flow of money to the Kremlin's death machine has grown stronger."
She argued that the UK government should have taken similar action "months ago" and urged ministers to bring forward a proposed ban on all Russian fossil fuels.
The protesters reached the jetty at the Navigator Terminals site by dinghy late on May 15, then attached themselves to the structure, unfurling a banner reading "oil fuels war".
The tanker Andromeda was unable to dock and was turned around in the early hours the following day.