Talim, the fourth typhoon of the year, made landfall at 10:20 p.m. (1420 GMT) in Zhanjiang city of Guangdong province, with winds near its center clocked at a top speed of 136.8 kph (85 mph), according to Guangdong weather bureau.
Winds stronger than 150 kph would put Talim in the severe typhoon category, very rare for a typhoon this early in the rainy season.
Talim is expected to move at a speed of 20 km per hour northwest and into Guangxi region early on Tuesday, Guangdong weather bureau added.
The national forecaster has urged authorities in Guangdong and Hainan to be on standby to respond to the typhoon.
Markets in Hong Kong halted trade for the day as the city's observatory put up a storm signal for Talim, and all court hearings in the city were adjourned.
China's meteorological centre forecast gale force winds in seas near southern provinces and regions and exceptionally heavy rains of 250-280 mm (9.8-11 inches) on the southwestern coast of Guangxi and northern Hainan Island.
Parts of Guangxi were told to brace for flash floods through Tuesday.
Hainan's Marine Forecasting Station warned of waves of up of 6 m south of the tourist island, Xinhua reported.
Authorities in Hainan asked ships in nearby waters to return to port and ferry services between Hainan and Guangdong were suspended early on Sunday.
Zhuhai Jinwan Airport in Guangdong cancelled 43 inbound and 36 outbound flights on Monday, and Meilan International Airport in Hainan's capital city Haikou and Qionghai Boao Airport, also on the resort island, cancelled all flights, state media reported.
High-speed railway and suburban trains on Hainan were suspended as well, with routes connecting to the tourist island shut until Friday.
Haikou city will also suspend classes, work, flights, and park and business activities from noon Monday, requiring residents to stay home, state television CCTV reported. Emergency shelters were opened to the public, it said.
In Fujian, north of Guangdong, local authorities declared a rainstorm emergency, with rains and thunderstorms predicted to persist from the weekend in most counties and cities.
The Chinese meteorological centre said rainfall in several cities in southeastern Fujian and eastern Jiangsu provinces reached 200-300 mm (7.87-11.81 inches) within three hours early on Monday.
Even before Talim's arrival, passengers on a public bus in Jiangsu waded through knee-high water to get to their seats after the vehicle ploughed through a water-logged road, videos posted on social media showed.