Damage seemed minimal, with streets mainly calm and no reports yet of landslides and major flooding on Monday morning.
Most transport, including ferries, buses and trams, was suspended, while the city's mass-transit railway system was running at longer intervals.
Hong Kong's Observatory said the Typhoon 8 signal, its third-highest, would remain in place until at least 11 a.m. (0300 GMT). Businesses can reopen if the signal is lowered, but schools will be shut for the full day, the city's education bureau said.
Sustained winds of 101 kph (63 mph) were recorded near the city's international airport on Lantau Island on Monday morning, with maximum gusts of 151 kph.
Authorities also issued the Amber rainstorm signal, its lowest in a three-tier ranking, at 4:55 a.m.
Tapah made landfall over Taishan in China's southern Guangdong province at 8:50 a.m. local time, and it is expected to move away from Hong Kong gradually, the observatory said.
In the neighboring Chinese city of Shenzhen, schools have also been shuttered.