Waves up to 11 metres high off Guangdong's coast signalled Usagi was about to make landfall 140 kilometres away from Hong Kong yesterday.
It struck Shanwei at around 7.40pm yesterday, with maximum wind speed of 162km/h near its centre, according to a weather news portal affiliated to the China Meteorological Administration.
The mainland's National Marine Environmental Forecasting Centre had already issued a red alert, its strongest warning.
Guangdong had beefed up its disaster relief forces and more than 44,000 fishing vessels had been told to return to port, according to Xinhua.
But by midnight last night, at least three people were dead and a fourth was missing, Xinhua reported, citing local officials.
Television footage showed a crane that had come crashing down onto a road in Shanwei. Power supply in the city was also reported to have been affected.
Before Usagi had even hit land, the entire old city of Shantou, in Guangdong, had already flooded, with water as deep as 80 centimetres, the Nanfang Daily reported. Many parts of the city were also left without power.
By noon yesterday, officials closed the Shantou Bay suspension bridge and the Queshi Bridge. At the same time, all flights were cancelled in and out of Shantou, Zhuhai and Shenzhen.
Baiyun International Airport in Guangzhou had issued an orange alert and stepped up support and emergency measures, China News Services reported.
In Guangzhou, several train services to Hunan and Fujian and high-speed trains linking Guangzhou, Zhuhai and Shenzhen were suspended.
At noon yesterday, officials closed the Shantou Bay suspension bridge and the Queshi Bridge.