The Hangzhou Bay Bridge links Shanghai to the industrial city of Ningbo across Hangzhou Bay, cutting the driving distance between them by 120 kilometers.
The official opening was shown live on state television on Thursday. The official Xinhua News Agency said traffic would start running on the six-lane bridge later on Thursday.
The bridge is a cable-stayed structure built at a cost of USD 1.7 billion. Construction started in November 2003.
The 32.5 kilometer Donghai Bridge had been the previous longest sea-crossing structure, linking Shanghai to the massive Yangshan deep water port.
Some of the financing for the bridge came from private sources, a first for such a large infrastructure project in China.