In this study, composition and agronomic traits, including yield-related traits, were investigated and compared among 35 Korean landrace germplasm for two years (2019 and 2020). The contents of 12 isoflavones were measured using high-performance liquid chromatography, and agronomic trait data were collected according to the Rural Development Administration criteria. Comparing isoflavone composition between the two years, all isoflavones, except glycitein, were generally higher in 2019. The values of yield-related and major agronomic traits decreased in most accessions in 2020. ANOVA showed highly significant differences (p<0.001) in genotypes, years, and genotype-by-year interaction for all isoflavones, the number of pods per plant, and 100-seed weight. The number of seeds per pod, days to flowering, and days to maturity also showed significant differences among genotypes, as well as between the two years. Correlation analysis revealed that daidzin and genistin were positively correlated with their malonylated form. Malonyl daidzin was also positively correlated with malonyl genistin levels. The accessions with the least variation in the total isoflavone content were IT 153844 (907.5 µg/g, 898.6 µg/g) and IT 252252 (663.2 µg/g, 636.4 µg/g). Some of the evaluated landraces will be a promising genetic source for developing soybean varieties with high levels of environmental stability in isoflavone content and agronomic traits.