Xinhua News Agency said a tornado, formed under the influence of the typhoon, killed at least 12 people and seriously injured another six in Zhejiang province in eastern China.
Sepat hit the coast of Fujian province in southern China early Sunday morning, causing a landslide that killed two people and left one missing, Xinhua said.
Earlier, the government evacuated nearly 540,000 people in Fujian, 300,000 in Zhejiang and 70,000 in Guangdong province, Xinhua said, citing emergency officials.
Xinhua quoted one villager in Zhejiang as saying the storm "tore down my mom's old house in no more than one minute."
Sepat was moving north up the coast packing winds of about 75 miles per hour, Xinhua said.
Heavy rains caused by Sepat also led to a train station collapsing west of Fuzhou, Fujian's capital, causing the derailment of six carriages of a cargo train.
Sepat later weakened to strong tropical storm, Xinhua said.
Some air traffic was halted in Fujian but has now resumed, it said.
Earlier, the storm, named after a Malaysian fish, knocked out power, washed out roads and overturned cars as it cut an east-west swath across Taiwan. At least one person was killed in Hualien when a car overturned and fell into a steep valley.
Sepat left three people dead and affected 542,000 people, mostly due to flooding, last week in the northern Philippines.
More than 3,500 people remained in evacuation shelters in the Philippines on Sunday due to the typhoon, which damaged $435,000 worth of crops, fisheries and roads, according to the government's National Disaster Coordinating Council.