The deaths occurred in Guangdong province, where Typhoon Usagi struck Sunday evening after veering away from the neighboring and densely populated financial hub of Hong Kong. It had earlier passed by Taiwan and the Philippines, where two deaths were reported.
The Guangdong provincial government's news office said on its official microblog that 13 of the 21 people were killed in Shanwei city, near the typhoon's landfall.
The official Xinhua News Agency had earlier reported three deaths - two killed when strong winds brought down a tree ahead of the typhoon's arrival, and a third person killed by falling window glass.
One county's electricity and water supply was cut off and houses were toppled by strong winds, Xinhua said. At one gas station near Shanwei city, winds blew cars off the road, it said.
In Hong Kong, dozens of trees were reported down and 13 people had sought medical treatment and seven of these were admitted to hospital, according to the Hong Kong government's information services department.
Usagi - Japanese for rabbit - was classified as a severe typhoon and had sustained winds of 175 kilometers (109 miles) per hour, with gusts of up to 213 kph (132 mph), on Sunday evening.
The storm had been a super typhoon on Saturday when it passed through the Luzon Strait separating the Philippines and Taiwan, a path likely sparing both places from the most destructive winds near its eye.
In the Philippines, Usagi left at least two people dead and two others missing, while in Taiwan nine people were hurt by falling trees.
The typhoon landed near the city of Shanwei in the Chinese province of Guangdong, about 140 kilometers (87 miles) northeast of Hong Kong, and was moving west-northwest at 22 kph (14 mph), the Hong Kong Observatory said late Sunday.
Intercity trains including the high-speed rail to Beijing, Shenzhen and Hong Kong would remain suspended until Tuesday, Xinhua said.
Ferry services between Hong Kong and nearby Macau and outlying islands were suspended as the observatory reported winds as strong as 68 kph (42 mph) and warned that a storm surge and heavy rains could cause flooding in low-lying areas.
Police in Shanwei ordered more than 8,000 fishing boats to return to port and more than 1,200 residents were taken to temporary shelters, China's official Xinhua news agency reported.
The typhoon wreaked havoc on airport schedules in Hong Kong, nearby Macau and mainland China, upsetting travel plans for many passengers who were returning home at the end of the three-day mid-autumn festival long weekend.
Hong Kong International Airport said 370 arriving and departing flights were canceled and another 64 delayed. Two of Hong Kong's biggest airlines, Cathay Pacific Airways and Dragonair, canceled flights to and from the city's airport starting at 6 p.m. Sunday. Cathay Pacific said Monday it would resume flights at noon, weather and safety conditions permitting.
Beijing-based Air China scrubbed 148 flights to and from Hong Kong, Macau and five nearby mainland cities. China Southern Airlines, based in Guangzhou, canceled all flights to and from Hong Kong and three mainland airports, Xinhua said.
Fujian province suspended shipping between mainland China and Taiwan, the news agency said.
Authorities in Guangdong initiated an emergency response plan for the Daya Bay nuclear power station northeast of Hong Kong as Usagi approached, ordering four of six reactors to operate at a reduced load, Xinhua said.
In Taiwan, more than 3,300 people were evacuated from flood-prone areas and mountainous regions. Rail service was restored Sunday on a rail line that had been buried by a landslide.
Another landslide late Saturday in the southeastern hot springs resort village of Chihpen sent mud and rocks crashing through the ground floor of a resort spa, forcing the evacuation of frightened guests. The Chihpen River breached its levies upriver, turning the village's main street into a rock-strewn stream, flooding homes and damaging vehicles.
In the Philippines, two people drowned and two went missing when a passenger boat capsized in rough waters off northeastern Aurora province, the Office of Civil Defense said Saturday. Nine passengers and crew were rescued.
The typhoon set off landslides and flooded parts of six Philippine provinces, but additional casualties were not reported.
Associated Press writers Kelvin Chan in Hong Kong and Jim Gomez in Manila, Philippines, contributed to this report.