According to the state data agency Statistics Korea, the number of women in 2015 will reach 25.31 million compared to 25.3 million men.
The shift is the consequence of a low birth rate and the natural longer life expectancy of women in a country undergoing a sharp generational shift.
South Korea became an ageing society in 2000 and is expected to become a "super-aged society" by 2026, when more than 20 per cent of its population will be aged over 65.
South Korea's working-age population between 15 and 64 will peak in 2016 at 37.04 million before starting to dwindle the following year, Statistics Korea said.
The most economically-active segment those between 25 and 49 already dropped for the first time last year to 19.78 million.