Finding that carbon emissions can be considered a sea pollutant, the court said countries had an obligation to take measures to mitigate their effects on oceans.
The countries that brought the case called the court decision "historic", and experts said it could be influential in shaping the scope of future climate litigation involving greenhouse gas (GHG) emissions.
"Anthropogenic GHG emissions into the atmosphere constitute pollution of the marine environment" under the international UNCLOS treaty, the International Tribunal for the Law of the Sea (ITLOS) ruled in an expert opinion.
Polluting countries therefore have "the specific obligation to take all measures necessary to ensure that... emissions under their jurisdiction or control do not cause damage by pollution to other states and their environment", the court said.
The case was brought in September by nine small countries disproportionately affected by climate change, including Antigua and Barbuda, Vanuatu and Tuvalu.
They asked the Hamburg-based court to issue an opinion on whether carbon dioxide emissions absorbed by the oceans could be considered pollution, and if so, what obligations countries had to address the problem.
The UNCLOS treaty binds countries to prevent pollution of the oceans, defining pollution as the introduction of "substances or energy into the marine environment" that harms marine life.
But it does not spell out carbon emissions as a specific pollutant, which the plaintiffs had argued should qualify.
- 'Fighting for survival' -
The court's opinion is advisory and non-binding but will influence how the UN treaty is interpreted around the world.
"This is the first-ever decision by an international tribunal on climate change and the oceans and clarifies the legally binding obligations of 169 countries that are party to the (UNCLOS treaty)," the nine plaintiff countries said in a statement.
The prime minister of Antigua and Barbuda, Gaston Browne, said small island nations were "fighting for their survival".
"Some will become uninhabitable in the near future because of the failure to mitigate greenhouse gas emissions. We demand that the major polluters respect international law, and stop the catastrophic harm against us before it is too late," he said.
The other island nations joining the ITLOS case were the Bahamas, Niue, Palau, St Kitts and Nevis, St Lucia, and St Vincent and the Grenadines.
The case is seen as the first major international climate justice case involving the world's oceans, and experts say it could have far-reaching implications for countries' future climate change obligations.
The Center for International Environmental Law said the case was "particularly significant" because it was the first of three key international court advisory opinions on climate change.
"For the first time, an international court has recognised that the fates of two global commons -- the oceans and the atmosphere -- are intertwined and imperilled by the climate crisis," said CIEL attorney Joie Chowdhury.
The other climate case rulings, by the Inter-American Court of Human Rights and the International Court of Justice (ICJ), are due to be given in the coming months.
- Rising temperatures -
Mandi Mudarikwa, Amnesty International's head of strategic litigation, said the ruling was "likely to inform future climate justice cases in national, regional and international courts".
Tom Mitchell, executive director at the International Institute for Environment and Development, told AFP the tribunal's opinion "is an important marker on the legal responsibility for the effects of climate change, which will doubtless be influential in shaping the scope and direction of future climate litigation".
Ocean ecosystems create half the oxygen humans breathe and limit global warming by absorbing much of the carbon dioxide emitted by human activities.
But increasing emissions can warm and acidify seawaters, harming marine life and ecosystems.
Rising global sea temperatures are also accelerating the melting of polar ice caps and increasing sea levels, posing an existential threat for small island nations.
Global sea surface temperatures hit a monthly record in April for the 13th month in a row, according to the EU's Copernicus Climate Change Service.
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