The Boeing plane will be based in Singapore for a week.
American P-8s already operate from Japan and the Philippines, and surveillance flights have also taken off from nearby Malaysia.
The agreement, which also covered co-operation on counter-terrorism, fighting piracy, and disaster relief, was announced after a meeting in Washington on Monday between US Defense Secretary Ash Carter and Singapore Defence Minister Ng Eng Hen.
Further P-8 deployments to the South East Asian city-state are expected.
China claims almost the entire South China Sea, putting it in territorial dispute with several countries, including Vietnam, the Philippines and Malaysia, among others. It has been building islands, equipped with runways, at breakneck pace.
China also has a separate dispute with Japan over islands in the East China Sea.
