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.
What’s so special about this bridge?
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.
Why has it been built?
It is part of China’s plan to create a Greater Bay Area, including Hong Kong, Macau and nine other cities in southern China.
The area is currently home to 68 million people.
In the past, travelling between Zhuhai and Hong Kong would take up to four hours – the new bridge cuts this down to 30 minutes.
Can anyone drive across the bridge?
No. Those who want to cross the bridge must obtain special permits, allocated by a quota system. And all vehicles will pay a toll.
The bridge is not served by public transport, so private shuttle buses will also ply the route. There is no rail link.
Authorities initially estimated that 9,200 vehicles would cross the bridge every day. They later lowered their estimations after new transport networks were built in the region.
Is it going to make up its costs?
The bridge, surrounding link roads and artificial islands cost a staggering $20bn to build – the main bridge alone cost $6.92bn.
Chinese officials say it will generate up to 10 trillion yuan ($1.44tn; £1tn) for the economy, but one Hong Kong lawmaker cast doubt on that figure.