The bridge which spans 55km (34 miles) and connects Hong Kong to Macau and the mainland Chinese city of Zhuhai is designed to withstand earthquakes and typhoons and it was built using 400,000 tonnes of steel, enough to build 60 Eiffel Towers.
About 30km of its total length crosses the sea of the Pearl River delta. To allow ships through, a 6.7km section in the middle dips into an undersea tunnel that runs between two artificial islands.
The bridge has cost about $20bn and seen several delays. Construction has been dogged by safety issues and reportedly at least 18 workers have died on the project.
Chinese leader Xi Jinping attended the opening ceremony of the bridge which took place in Zhuhai along with the leaders of Hong Kong and Macau on Tuesday.
In the past, travelling between Zhuhai and Hong Kong would take up to four hours - the new bridge cuts this down to 30 minutes.
The bridge lies in a region known as China's Greater Bay Area, which boasts a population of about 67 million and a GDP of $1.5 trillion, on par with that of South Korea, making it one of China's richest regions.