The case at the tribunal in The Hague was brought by the Philippines, which argues Chinese activity in the region is against international law.
China claims about 90% of the South China Sea, including reefs and islands also claimed by others.
China says it does not recognise the tribunal and has refused to take part.
The case is being decided by an arbitration tribunal under the United Nations Convention on the Law of the Sea (UNCLOS), which both countries have signed.
The ruling is binding but the tribunal has no powers of enforcement.
However, observers say it could favour the Philippines – and China risks reputational damage if it does not abide by it.
They also warn there is a risk that China could react aggressively to a ruling against it.
The US has sent an aircraft carrier and fighter jets to the region in anticipation of this, prompting an angry editorial in the Global Times, a strongly nationalist state-run newspaper, calling for the country to prepare for “military confrontation”.
Meanwhile the Chinese Navy has been carrying out exercises near the disputed Paracel islands.
The tribunal hearing the case has previously said it is the appropriate body to rule on at least seven of the 15 claims in the Philippines’ case and was still considering the other eight.
Beijing has been trying to gather international support for its view that the tribunal’s ruling should be rejected.
Chinese diplomats have written a slew of articles setting out their government’s position in English-language media around the world.
China says about 60 countries support its stance that the tribunal’s ruling should be rejected, but few have declared their support publicly. (Courtesy BBC)