Out of 13.7 million singles aged 15 and older, 2.82 million people in their 30s were single last year, up from 2.68 million five years earlier, according to the 2020 census by Statistics Korea.
The percentage of unmarried people in their 30s reached 42.5 percent last year, up 6.2 percentage points from 36.3 percent in 2015, the data showed.
It marked the first time for the number to top 40 percent for the age group.
By gender, 50.8 percent of men in their 30s remained unmarried, while single women accounted for 33.6 percent.
Many young South Koreans are opting to distance themselves from life's three major milestones -- dating, marriage and having children -- because they cannot find decent jobs amid a protracted economic slowdown and skyrocketing home prices.
Accordingly, the country is struggling with a sustained fall in childbirths.
Last year, South Korea's total fertility rate -- the average number of children a woman bears in her lifetime -- hit a record low of 0.84, much lower than the replacement level of 2.1 that would keep South Korea's population stable at 51 million.
The statistics agency said on top of the underlying trend of a delay in marriage, the COVID-19 pandemic may also have affected marriage data.
The census showed women with higher education backgrounds got married less than others, with 22.1 percent of women with master's degrees being single last year.
The percentage of single people made up 31.1 percent of people aged 15 or over last year, down from 31.3 percent five years earlier as the number of teens sharply declined amid a sustained fall in childbirths.
The proportion of married people accounted for the largest share of 55.9 percent, and that of divorced people made up 5.8 percent, up 0.7 percentage point from five years earlier.
Meanwhile, the number of households that raised pets reached 3.13 million last year, accounting for 15 percent of the total.
The data showed 2.4 million households raised dogs, followed by 717,000 households with cats. (Yonhap)