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.
(Excerpts / pics : BBC News)