This study investigated the effects of seasonal variation on carcass traits, primal cut yields, and carcass grade distribution in approximately 700,000 Landrace × Yorkshire × Duroc (LYD) pigs slaughtered in Korea. Carcass weight, backfat thickness, and the yields of five major primal cuts were measured using the VCS2000 non-destructive carcass analysis system and analyzed across seasons. All primal cuts exhibited similar seasonal production patterns, with the highest yields observed in winter and the lowest in summer. Z-score standardization revealed that primal cut yields decreased during late summer and early fall and increased during late fall and winter. Seasonal differences were also observed in carcass grade distribution, with the proportion of the highest grade (1+) decreasing during summer and increasing during winter. Across all carcass grades, carcass weight, backfat thickness, and primal cut yields were consistently greater in winter. Spearman correlation analysis showed weak but significant associations between ambient temperature and carcass grade distribution, with the proportion of grade 1 + negatively correlated with temperature, whereas grade 2 exhibited a positive correlation. Logistic regression analysis demonstrated that carcass weight and backfat thickness were the most influential variables affecting carcass grade classification, while seasonal and environmental factors provided additional explanatory power. Overall, these results suggest that seasonal environmental conditions are associated with variations in carcass composition, primal cut yield, and carcass grading outcomes in Korean LYD pigs. These findings provide useful insights into seasonal patterns in pork production and may help improve management strategies and carcass grade prediction under varying environmental conditions.