The Arbitral Tribunal at The Hague had strong words for how China's actions have affected the marine environment.
In the past two years, China has dredged rock and sand to transform seven reefs in the South China Sea's Spratly chain and constructed port facilities, military buildings and airstrips.
The tribunal said three independent experts on coral reef biology assisted it in evaluating the available scientific evidence and the Philippines' expert reports.
It found that China's large-scale land reclamation and construction of artificial islands on seven features in the Spratlys "caused severe harm to the coral reef environment".
According to its ruling, "China has violated its obligation under Articles 192 and 194 of the Convention (United Nations Convention on the Law of the Sea) to preserve and protect the marine environment with respect to fragile ecosystems and the habitat of depleted, threatened, or endangered species".
The tribunal also found that the Chinese authorities were aware that Chinese fishermen have harvested endangered sea turtles, coral and giant clams on a substantial scale in the South China Sea, "and had not fulfilled their obligations to stop such activities".
The marine ecosystems in the region are key to maintaining world fish stocks and biodiversity.
Experts have warned that some fish species are close to extinction due to overfishing.