BEIJING—As Beijing intensifies efforts to sway U.S. public opinion, a Chinese organization focused on the disputed South China Sea has set up what its founders say is China’s first think tank inside the Washington Beltway.

The Institute for China-America Studies’ three staff members work from a small office near Ronald Reagan International Airport. Their mission is to research and conduct exchanges on maritime issues and China-U.S. relations, not to represent the Chinese government, according to its executive director, Hong...