Paul Reichler said he also believed that in spite of China's rejection of the case, international pressure would eventually oblige Beijing to comply with a ruling against it.
In a legal setback for Beijing on Thursday, the Hague-based Permanent Court of Arbitration ruled that it has jurisdiction to hear some territorial claims the Philippines has filed against China over disputed areas in the South China Sea.
Reichler called this a "major victory" and said he expected an oral hearing on the merits of the Philippines' case would be held by the end of the year and a final ruling made within six months of that.
"We are talking about a final judgment being issued right in the middle of 2016, perhaps June of 2016, but I am only speculating at this point," he said.
"For the Philippines, having now established that there is jurisdiction for the tribunal to consider its claim ... leaves us very optimistic that the ultimate outcome will be very successful," Reichler said.
Manila filed the case in 2013 to seek a ruling on its right to exploit the South China Sea waters in its 200-nautical mile exclusive economic zone as allowed under the United Nations Convention on the Law of the Sea (UNCLOS).
China has boycotted the proceedings and rejects the court's authority in the case. Beijing claims sovereignty over almost the entire South China Sea, dismissing claims to parts of it from Vietnam, the Philippines, Taiwan, Malaysia and Brunei.
Reichler said it was "inevitable" that China would one day have to seek some accommodation with a judgment against it if it wanted to be seen as a state that respected international law.
