M503 is a flight route for civil aviation located off the southeast coast of China. It joins with three extension routes, W121, W122, and W123, which connect M503 with Chinese coastal cities. The route is a point of contention between China and Taiwan, as it passes very close to the median line of the Taiwan Strait. This brings planes on the route very close to Taiwanese-controlled airspace.
Originally proposed in 2015 by the Civil Aviation Administration of China (CAA) to reduce congestion along existing flight routes, M503 was shifted further from the median line and opened only to southbound flights following negotiations between China and Taiwan. In 2018, China began to allow northbound flights without agreement from Taiwan. In 2024, M503 was shifted back to its original route, at one point just 4.2 nautical miles (7.8 km; 4.8 mi) from the median line.