Objectives: To determine the trend of monozygotic (MZ) and dizygotic (DZ) twinning in Korea from 1991 to 2018. Methods: This study used twin birth certificate data (1991-1994 years: 13,527 pairs, 2000-2018 years: 115,656 pairs), provided by Statistics Korea. Weinberg’s differential rule was used to estimate the number of MZ and DZ twin births. The impact of assisted reproductive technologies (ART) on MZ and DZ twinning was analyzed by examining data after the introduction of the national support program for infertile couples. The odds ratio (OR) and 95% confidence intervals were calculated to describe the trend in the MZ and DZ twinning ratio. Results: From 1991 to 2018, the DZ/MZ ratio increased significantly from 0.45 to 3.30. Prior to the introduction of the national support program for infertile couples (1991-1994 years), the DZ/MZ ratio was 0.54, but after its introduction, the DZ/MZ ratio was 1.99 in 2005-2009 (OR: 3.69, 95% confidence interval: 3.53-3.85), 2.53 in 2010-2014 (14.69, 4.49-4.89), and 3.02 in 2015-2017 (5.61, 5.37-5.87). Conclusions: Over the past three decades, twin birth and proportion of DZ twining have risen in Korea, due to the widespread use of ART. There is a need to understand the factors contributing to DZ to MZ twining birth whether twining birth originated spontaneously or from assisted-conception.