The United States and Taiwan enjoy a robust unofficial relationship and close cooperation on a wide range of issues. Maintaining strong, unofficial relations with Taiwan is a major U.S. goal, in line with the U.S. desire to further peace and stability in Asia. The 1979 Taiwan Relations Act provides the legal basis for the unofficial relationship between the United States and Taiwan, and enshrines the U.S. commitment to assist Taiwan in maintaining its self-defense capability. The United States believes that cross-Strait differences should be resolved peacefully in a matter acceptable to people on both sides of the Strait, opposes unilateral changes to the status quo by either side, and encourages both sides to engage in constructive dialogue on the basis of dignity and respect.
The United States changed its diplomatic recognition from Taipei to Beijing on January 1, 1979. In the 1979 U.S.-PRC Joint Communique, the United States recognized the Government of the People’s Republic of China as the sole legal government of China. The Joint Communique also stated that the people of the United States will maintain cultural, commercial, and other unofficial relations with the people of Taiwan. The American Institute in Taiwan (AIT) is responsible for implementing U.S. policy toward Taiwan.
Source: https://www.state.gov/u-s-relations-with-taiwan/