The objective of this study was to determine the interaction effects of photoperiod management and dietary grain allocation on the productivity of lactating dairy cows. Sixty Holstein cows in mid-lactation (days in milk = 113 ± 36.0; mean ± SD) were assigned to either a long photoperiod (LP; 16 h light) or a short photoperiod (SP; 8 h light) treatment. After a 30 d light adaptation period, cows within each photoperiod treatment were fed three diets differing in the grain content (15%, 25%, and 35% of dietary dry matter) in a 3 × 3 Latin square design. Cows exposed to the LP increased milk yield compared with those exposed to the SP (39.0 vs. 36.8 kg d⁻¹) after a 30 d of light adaptation period. Although the positive effect of LP was not sustained after cows were assigned to dietary treatments in a 3 × 3 Latin square design, cows fed the 35% grain diet increased fat-corrected milk yield compared with those fed 25% or 15% grain diet (35.9 vs. 33.4 or 32.9 kg d⁻¹, respectively). This study indicated that LP management and feeding high grain diets did not lead to synergistic effects on productivity of dairy cows.