This study examines the distribution of semantic correspondences between Sino-Japanese and Sino-Korean words among the 20,000 most frequently used words in Japanese. To achieve this, a Database of Sino-Japanese Vocabulary in Contrast to Korean was created and quantitatively analyzed. Among the top 20,000 Japanese words (as listed in the Vocabulary Database for Reading Japanese by Matsushita, 2011), 46.4% are Sino-Japanese words composed of two or more characters. Notably, 93.4% of these words are cognates shared between the two languages, which represents 43.3% of the total vocabulary. Furthermore, 98.1% of the shared cognates carry the same basic meaning in both languages, while only 1.9% exhibit semantic differences that require careful consideration. Interestingly, the proportion of synonymous cognates in modern Japanese and Korean (98.1%) is higher than the 86.5% reported by Lee (2005) and exceeds the 83% for Japanese–Chinese synonymous cognates according to Matsushita et al. (2020) (83%). Additionally, when examining correspondences among Japanese, Korean, and Chinese, over half of the Sino-lexical items (55.6%) are shared synonymous cognates across all three languages, highlighting the strong historical lexical connections among them. The close semantic correspondence between Japanese and Korean may be attributed to their geographical and historical proximity. These findings suggest that Korean learners of Japanese could understand approximately 40% of Japanese vocabulary through sound-character correspondences between the two languages. Consequently, vocabulary instruction for Korean learners can be improved by systematically emphasizing cognates and introducing them early in the learning process to facilitate lexical acquisition. Finally, the database developed in this study can be utilized not only for retrieving information about Sino-Japanese and Sino-Korean words and for cognate-based learning but also for integration into web tools, such as vocabulary frequency profilers.