The Hangzhou Bay Bridge links Shanghai to the industrial city of Ningbo across Hangzhou Bay, cutting the driving distance between them by 120 kilometres.
It will cut the driving distance between the two key cities, in China’s prosperous Yangtze River delta, by 75 miles and reduce the journey time by one and a half hours.
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, but trucks will be barred.
The bridge is a cablestayed structure built at a cost of $ 1.7 billion.
Construction started in November 2003.
Some of the financing for the bridge came from private sources, a first for such a large infrastructure project in China.
“The bridge has become well-known and is expected to attract many visitors,” Mr Jin Jianming, the deputy chief commander of the bridge project, told the state-run Xinhua news agency.
The concrete and steel structure, built to withstand typhoons and shifting soil conditions, is long enough to link the UK and France across the English Channel.
China is spending heavily on infrastructure to maintain its double digit rates of economic growth.
The 32.5 kilometre Donghai Bridge had been the previous longest seacrossing structure, linking Shanghai to the massive Yangshan deep water port.