Pete Bethune has received a two-year suspended sentence and banned from Japan for five years, for obstructing a whaling fleet in the Antarctic Ocean.
He is expected to be deported back to New Zealand on the next available flight.
Bethune, 45, was also found guilty of assault, for throwing a bottle of rancid butter at whalers on board the whaling ship Shonan Maru II.
The ruling was made at the Tokyo District Court this afternoon, after Bethune obstructed the activities of a Japanese whaling fleet in February.
The sentence is suspended for five years, meaning Bethune will not be jailed.
3 News reporter Melissa Davies was in court for the sentence, watch her report.
The activist had hoped to receive a suspended sentence, according to his wife Sharyn in Auckland, but has said he was "very nervous" ahead of sentencing.
"I will accept the verdict, but I do hope to go home soon," Peter Bethune, a former member of the Sea Shepherd Conservation Society, told Kyodo News agency in an interview at Tokyo Detention House.
He pleaded guilty last month to charges relating to the illegal boarding of a Japanese whaling ship, but not guilty to assault, after being held in custody since February, when he boarded the Japanese whaling fleet's security ship the Shonan Maru II during its annual trip south.
Sharyn Bethune has said she hopes her husband will be able to return to New Zealand - and his daughters Danielle and Alycia - as early as Saturday morning.
Sharyn Bethune reacts to verdict
A Ministry of Foreign Affairs spokesman told NZPA a New Zealand government official would attend the sentencing.
Bethune was captain of the US-based environmentalist group's futuristic powerboat, the Ady Gil - his former record-setting trimaran Earthrace - which sank after an earlier collision with the Shonan Maru II.
He tried to serve a citizen's arrest warrant to the captain of the Shonan Maru II over the sinking of his vessel and the attempted murder of him and his crew, but was captured.
"I deeply regret" if someone was injured in the Antarctic Ocean, he said ahead of his ruling tomorrow. "I have nothing against Japanese."
At his trial in Tokyo, Bethune pleaded guilty to charges including trespassing, vandalism and holding a knife, which he used to cut netting as he climbed onto the ship from a jet ski, but he has denied the assault charge.
He was also accused of causing chemical burns to the face of a whaler with a rancid butter stink bomb and four other charges.
"What I was doing was standing up for New Zealand and Australia," he told Kyodo.
He stressed that it was "deeply offensive" for the two countries to see the Japanese hunting whales specifically in the Antarctic Ocean, and that he "wouldn't mind if it is Northern Pacific".
Bethune's Sea Shepherd crew members say they are "relieved" with the sentence.
Sea Shepherd founder Paul Watson responds to verdict.
The Sea Shepherd group has said Bethune would no longer join
its actions after taking a bow and arrows on the latest high-seas
campaign, contrary to Sea Shepherd's stance of "aggressive but
non-violent direct action".