Alex Harris, 27, who lives in Australia but whose family lives in Devon, received the sentence.
Frank Hewetson, 45, a father-of-three from London, has been jailed for three days but Greenpeace say he can also expect to be inside prison for two months.
They were among a 29 Greenpeace activists, including six Britons and a photographer who were held last week when armed Russian officials boarded the Arctic Sunrise near Gazprom's oil platform.
The ship and protesters have been taken to the port of Murmansk, and Russian authorities said they were investigating charges of piracy, which carries a jail term of 10 to 15 years, against the activists.
Peter Willcox, captain of the Arctic Sunrise has also been jailed along with a freelance photographer.
John Sauven, executive director of Greenpeace UK said they were "deeply shocked by the severity of the Russian judge's decision."
He added: "Every indication now is that the remaining members of Greenpeace, including a further four Britons, will receive the same two month sentence.
"The decision is a total over-reaction to a peaceful protest that was drawing attention to the dangers of Russian oil giant Gazprom drilling in the Arctic Ocean.
"Our focus now is on supporting the families of the UK activists being held and galvanising public opinion to bring pressure on the Russian authorities to release all those detained.