The number of people of working age in Asia's fourth-largest economy fell for the first time ever last year, South Korea's official statistics agency said Monday.
In the decades after the Korean War the South propelled itself from a devastated ruin to the world's 11th-largest economy and a member of the OECD club of advanced nations.
But it faces looming demographic challenges with a rapidly ageing population.
The country has one of the world's lowest birth rates as people marry and have children later, amid worries over costs and as women look to focus on their careers.
In an annual census, the working-age population, defined as those aged 15 to 64, fell by 116,000 in 2017 to 36.2 million, Statistics Korea said.
It was the first time the figure had fallen, it added.
The total population rose to 51.4 million, up 0.3 percent, with 14.2 percent of people aged 65 and over.
Earlier this month figures showed births plummeting 12 percent in 2017 to 357,771, an all-time low.
The fertility rate -- the number of children a woman can be expected to have in a lifetime -- also dropped to a record low of 1.05. The rate needed to keep a population stable is 2.1.