Former Australian Greens leader Bob Brown will direct the coming campaign against Japanese whaling in the Southern Ocean, taking over from the group's founder Paul Watson.
Dr Brown joined the activist group's board as a director on January 1 following his retirement from the Greens leadership and the Australian Senate last year.
He is a long-time friend of Mr Watson, a Canadian who has until now captained Sea Shepherd's flagship Steve Irwin in the group's war against Japanese whalers in the Southern Ocean.
The group has been engaged in heated confrontations with the whaling fleet and was ordered by a US court in December not to approach the Japanese ships, overturning an earlier decision in favour of Sea Shepherd.
Mr Watson on Tuesday resigned as president of Sea Shepherd's US chapter and the Australian president as a result of legal proceedings against him and the US court's injunction ordering protesters to stay at least 450 metres from the whaling fleet.
Dr Brown was appointed as a director in January and will spearhead the anti-whaling campaign Operation Zero Tolerance in the Southern Ocean this season alongside Australian director Jeff Hansen.
Current board member and adviser to Mr Watson, Marnie Gaede, will assume the role of president of Sea Shepherd in the US.
Dr Brown's new role will involve directing Sea Shepherd's ninth campaign in the Southern Ocean.
"I am honoured to serve the great whales of the Southern Ocean and Sea Shepherd in this way," Dr Brown said.
"My admiration for Paul Watson is inversely proportional to the Japanese government's anger at Sea Shepherd's success at preventing the slaughter of almost 4000 whales in recent years."
Former Liberal politician Ian Campbell joined Dr Brown in Hobart on Tuesday in support of Sea Shepherd.
"The Japanese have armed their vessels with harpoons that maim and kill whales and guns that can maim and kill humans," the former federal environment minister said.
"Sea Shepherd Australia is there to protect whales and if its vessels have to get within 450 metres of a killer ship to save a whale, then that is what will be done."
If the Japanese said they wouldn't go within 450 metres of a whale, Sea Shepherd Australia would do the same, Mr Campbell said.
Mr Watson is set to defy the US court order and is in the Southern Ocean waiting to tail the fleet from the Japanese Institute of Cetacean Research (ICR), which conducts the so-called scientific whaling program and initiated the US court action.
He is the subject of two Interpol "red notices", seeking to extradite him to Costa Rica and Japan to be put on trial for his actions in anti-whaling campaigns.
The Steve Irwin will be captained by Siddharth Chakravarty, but Mr Watson will remain on board to document this year's campaign.
The Japanese fleet reportedly left for Antarctica in late December.
Sea Shepherd claims the ICR lawsuit against it is funded by a Japanese government subsidy of some US$30 million in relief funds donated to help Japan's tsunami victims, not to hunt whales.