23 October 2018 09:38 am Views - 8734
Including its access roads, the bridge spans 55km (34 miles) and connects Hong Kong to Macau and the mainland Chinese city of Zhuhai.
The bridge has cost about $20bn (£15.3bn) and seen several delays.
Construction has been dogged by safety issues - at least 18 workers have died on the project, officials say.
Mr Xi attended the opening ceremony of the bridge which took place in Zhuhai along with the leaders of Hong Kong and Macau on Tuesday.
The bridge will open to regular traffic on Wednesday.
The crossing connects three key coastal cities in southern China - Hong Kong, Macau and Zhuhai.
The bridge, designed to withstand earthquakes and typhoons, 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 remaining sections are link roads, viaducts and land tunnels connecting Zhuhai and Hong Kong to the main bridge.
It is part of China's plan to create a Greater Bay Area, including Hong Kong, Macau and nine other cities in southern China.
In the past, travelling between Zhuhai and Hong Kong would take up to four hours - the new bridge cuts this down to 30 minutes. (BBC)