President Kim Dae-jung called an emergency cabinet meeting to put the government on disaster alert as the country struggled to cope with the effects of torrential rain and lethal winds from Typhoon Rusa.
About 50 people are known to have died and many more are missing in floods and landslides that have also caused massive disruption to South Korea's transport network.
As the storm finally moves away from the Korean peninsula, troops are being sent to help in any rescues and repair damage to the nation's infrastructure including flooded roads and railways and power outages to hundreds of thousands of homes.
Local media are predicting that the number of dead will rise further as the authorities assess the full impact of Rusa - the worst storm to hit the Korean since Typhoon Sarah in 1959.
South Korea's National Disaster Prevention Headquarters said floods washed away seven people and landslides buried 10.
Some people were killed in collapsing houses and in strong winds.
Rusa's wrath
Winds hit 204km/h (127mph)
89 centimetres (35 inches) of rain
1.2 million people without power
20,000 homes flooded
Flights cancelled
Roads and railways closed
Ferries suspended
5,100
hectares (12,600 acres) of farmland flooded
The eastern port city of Gangneung was among the worst hit.
Landslides there buried buildings and cars and pedestrians waded through waist-high water.
Many of the deaths occurred when rain caused landslides which buried four homes and 10 cars in Kangnung.
Two people were also killed in South Korea's south coast town of Keochang when strong winds sent a heavy signpost crashing down on their car, national officials said.
US marines missing
Earlier, two US marines stationed on the southern Japanese island of Okinawa were reported missing after they were swept into the sea by massive waves whipped up by the storm.
Japanese coastguard officials said the two went missing after going out to watch the waves on Friday night.
A third marine swam to shore, sustaining only minor cuts.