The New Zealand actor has been signed on as the Antarctic Ocean Alliance's latest ambassador.
The Alliance is made up of about 30 international organisations working together to establish the world's largest marine protected area.
It launched its bid in February this year, and about 80,000 people have signed a call for protection of 19 key marine habitats including the Ross Sea and East Antarctica.
Neill said some parts of the world were too precious to lose, and should be free from human exploitation.
"We are sending a strong message to decision makers that Antarctica's marine environment and the Ross Sea, our 'last great ocean' must be preserved for all time, for those that come after us and for our fragile planet."
