The death toll included a pilot who died after a helicopter crashed on Wednesday during efforts to contain wildfires in the southeastern town of Uiseong, one of the hardest-hit areas.
The aircraft had no other crew members.
The National Fire Agency said at least 26 other people sustained varying degrees of injuries.
An ancient Buddhist temple, houses, factories and vehicles were destroyed in the wildfires that have burned 175 square kilometres, the government's emergency response centre said.
In a televised address, Acting President Han Duck-soo said the wildfires that began last Friday were the worst so far.
"Damages are snowballing," Han said.
"There are concerns that we'll have wildfire damages that we've never experienced, so we have to concentrate all our capabilities to put out the wildfires in the rest of this week."
Han said crews were struggling to extinguish the wildfires because strong winds swept the areas overnight.
About 4650 firefighters, soldiers and other personnel were working on Wednesday with the help of about 130 helicopters.
Han said "a small amount" of rain was expected on Thursday.
As of Wednesday evening, firefighters were fighting at least four active wildfires, including in the southeastern coastal town of Yeongdeok, which alerted residents of the nearest village to evacuate to an indoor gymnasium.
Strong winds and smoke-filled skies forced authorities in the southeastern city of Andong to order evacuations in two villages, including Puncheon, which is home to the Hahoe folk village, a Unesco World Heritage Site founded around the 14th-15th century.
Hikers were advised to leave the scenic Jirisan Mountain, one of the country's largest national parks, as another fire spread closer.
Observers say the ongoing wildfires are the third biggest in South Korea in terms of land burned.
The largest fires were in Andong, the neighbouring counties of Uiseong and Sancheong, and the city of Ulsan.
On Tuesday, officials said firefighters had extinguished most of the flames from the largest wildfires in those areas, but wind and dry conditions allowed them to spread again.
The blaze in Uiseong destroyed nearly half of more than 30 structures at Gounsa, a temple which was said to be originally built in the seventh century.
Among the burned structures were two state-designated "treasures": a pavilion-shaped building erected on a stream in 1668, and a Joseon Dynasty structure built in 1904 to mark the longevity of a king.
The another state-designated "treasure", a stone Buddha statue reportedly manufactured in the eighth century, was moved to a safe place, according to government and Buddhist officials.
The Korea Forest Service said it had raised its wildfire warning to the highest level nationwide, requiring local governments to assign more workers to emergency response, tighten entry restrictions for forests and parks, and recommend that military units withhold live-fire exercises.
The 18 dead include four firefighters and government workers who died in Sancheong on Saturday after being trapped by fast-moving flames driven by strong winds, according to officials.
Government officials suspect human error caused several of the fires, possibly due to the use of fire while clearing overgrown grass in family tombs or sparks from welding work.
