From the early 1980s to 2007, spanning about thirty years, Korea’s sex ratio at birth remained higher than the natural ratio, having first exceeded it in the 1970s. This study arose out of concerns that those born during these years of imbalanced sex ratios at birth could, upon reaching reproductive age, engender imbalances in the sex ratio at marriage. For the analysis, I calculated the sex ratio of the current unmarried population, the hypothetical matching index for the unmarried population, and Schoen’s (1983) S-index. The results reveal that imbalances in the sex ratio at birth, while nonexistent in the early 1990s, worsened from the mid-2000s onward, to the extent that by 2021, there were 19.6 percent more unmarried men than unmarried women nationwide. Over the years, imbalances in the sex ratio at marriage have become much more severe in non-capital areas than in the capital region.