TOKYO—Japan’s military would consider joining U.S. forces in regular patrols in the South China Sea, the nation’s top uniformed officer said, underscoring how China’s territorial claims are encouraging Tokyo to play a greater role in regional security.

Adm. Katsutoshi Kawano, chief of the Joint Staff of the Japan Self-Defense Forces, said in an interview that China’s recent moves to build artificial islands have created “very serious potential concerns” for Japan, a trading nation that relies on the sea lane that runs through...