By Eun-Young Jeong

SEOUL -- South Korea's population decreased for the first time in 2020, entering a downward trend likely worsened by the pandemic.

South Korea, Asia's fourth-largest economy, reported a population of 51.8 million last year, a drop of 20,838 people from 2019, according to government data released Sunday, measuring citizen registration numbers.

Leading the population drop is South Korea's low number of births. Births have fallen to record lows every year since 2016. The average number of children a woman will bear in her lifetime in South Korea was 1.1 last year, the lowest fertility rate in the world, according to a United Nations Population Fund's survey of more than 200 countries.

Statistics Korea, South Korea's statistics arm, initially predicted the country's population would enter into decline in 2021 after accounting for other population-change factors, such as immigration.

But the pandemic, which hammered jobs and forced couples to delay marriages, is likely to have weighed on birth rates, especially in South Korea where the majority of child births occur after marriage, said Choi Seong-soo, a sociology professor at Yonsei University in Seoul.

"It's too early to see these factors impact birth rates now," Mr. Choi said. "But we're expecting to see the effects come through 2021 and 2022."

A Bank of Korea report released in December said the Covid-19 shock could have a permanent impact on the country's birth rates as the delays in childbearing turn into permanent decisions to forego children altogether.

South Korea recorded 275,815 births and 307,764 deaths in 2020, according to the Ministry of the Interior and Safety. The country, which has kept outbreaks relatively stable, has reported 981 deaths tied to Covid-19.

With fewer newborns, South Korea is projected to have the world's largest proportion of individuals 65 or older by 2045, according to Statistics Korea, overtaking Japan, currently one of the world's oldest nations.

Japan has made preschool free and urged companies to let employees leave on time. But after a mild upswing last decade, the country's birth rate has fallen in each of the past four years.

South Korea's aging population is expected to hamper the country's growth as its working-age population declines. One study estimates that South Korea's potential production capacity will drop by about one percentage point starting in 2024 because of demographic changes.

The pandemic has highlighted South Korea's growing reliance on low-income immigrant workers, especially in agriculture and basic manufacturing sectors, after air-travel shutdowns abruptly closed the doors to many temporary workers. Policy makers have started looking into immigration as a response to shifting population dynamics, with a focus on bringing skilled foreign workers into the country.

Government officials estimate immigrants and temporary foreign residents will make up 6.9% of the overall population by 2040, an increase from last year's 4.3%.

In South Korea, the government has poured in tens of billions of dollars to boost its ailing birth rate, which has been a priority policy for many South Korean leaders, including President Moon Jae-in.

In some smaller, rural cities outside of the Seoul metropolitan area, local governments have handed out cash to households with multiple children. One city is considering providing a housing-purchase stipend of about $47,000 for families with three or more children. The country's competitive housing lottery for buying new apartment properties -- a points-based system with a random draw -- is heavily skewed to benefit households with more children.

The efforts haven't produced much of a turnaround. Meanwhile, many young women have been put off by the government's diagnosis of sliding birth rates. In 2017, a study from a government-affiliated research agency singled out women's high academic and career achievements as the cause for declining birth rates.

Though South Korea's aging-population crisis isn't unique, demographers say its pace is unprecedented. About 14% of South Koreans currently are elderly. That is below the Organization for Economic Cooperation and Development's average, according to the group's most recent, 2018 data, roughly half Japan's level and below the 20% range seen in parts of Europe.

But in two decades, South Korea's elderly will exceed 40% of the total population, said Lee Sam-sik, director at Hanyang University's Institute of Aging Society, in Seoul.

"That's a huge jump in the elderly population in a very short period," Mr. Lee said.

--Peter Landers in Tokyo contributed to this article.

Write to Eun-Young Jeong at Eun-Young.Jeong@wsj.com

(END) Dow Jones Newswires

01-04-21 1104ET