Guangdong will invest 50 billion yuan (HK$47.2 billion) to build a bridge linking the province with Hainan island, local reports say.
The 30km structure is part of a national project to build five bridges starting in Tongjiang, part of northernmost Heilongjiang province, and ending in the Hainan resort town of Sanya, the reports said.
The other three bridges will be in Hangzhou bay, the Bohai Sea and across the Yangtze River mouth.
The cost of the entire scheme is 570 billion yuan.
The bridges are part of the national highway grid, which includes five north-south and seven east-west routes.
The Guangdong-Hainan route is regarded as the most challenging of the highways because of the number of bridges along its span.
A Guangzhou Academy of Social Sciences researcher welcomed the project, saying the bridges were essential if Guangdong was to expand its economy.