Officials said the deaths were caused by a landslide triggered by the storm. Ten others have been reported missing.
Lekima made landfall in the early hours of Saturday in Wenling, between Taiwan and China’s financial capital Shanghai.
The storm was initially designated a “super typhoon”, but weakened slightly before landfall.
The fatal landslide happened in Wenzhou, near where the storm made landfall, state media said. It occurred after a barrier lake – or natural dam – formed, pooling water from the heavy rains before collapsing.
Lekima is now slowly winding its way north through the Zhejiang province, and is expected to hit Shanghai, which has a population of more than 20 million.
Emergency crews have battled to save stranded motorists from floods and searched for survivors in the rubble of damaged buildings. Fallen trees and power cuts are widespread.
The storm is expected to weaken further by the time it reaches the city, but will still bring a high risk of dangerous flooding.
“Now the disaster zones are mainly in the rural-urban integration zones,” said Fu Songliang, head of a rescue team for Ningbo City’s fire service.
“These are low-lying areas. When the flooding from mountainous areas converges and comes down, these areas are quite seriously affected,” he told Reuters news agency.
Shanghai evacuated some 250,000 residents, with another 800,000 in Zhejiang province also being taken from their homes.
An estimated 2.7 million homes in the region lost power as power lines toppled in the high winds, Chinese state media said.
It is the ninth typhoon of the year, Xinhua news said – but the strongest storm seen in years. It was initially given China’s highest level of weather warning but was later downgraded to an “orange” level.
Chinese weather forecasters said the storm, which had winds of 187km/h (116mph) when it made landfall, was moving north at 15km/h.
It earlier passed Taiwan, skirting its northern tip and causing a handful of injuries and some property damage.
Coming just a day after a magnitude six earthquake, experts warned that the combination of earth movement and heavy rain increased the risk of landslides.
Lekima is one of two typhoons in the western Pacific at the moment.
Further east, Typhoon Krosa is spreading heavy rain across the Northern Mariana Islands and Guam. It is moving north-west and could strike Japan some time next week, forecasters said.