Stomatal movements are regulated by many environmental signals, such as light, CO2, temperature, humidity, and drought. Recently, we showed that photoperiodic flowering components have positive effects on light-induced stomatal opening in Arabidopsis thaliana. In this study, we determined that light-induced stomatal opening and increased stomatal conductance were larger in plants grown under long-day (LD) conditions than in those grown under short-day (SD) conditions. Gene expression analyses using purified guard cell protoplasts revealed that FT and SOC1 expression levels were significantly increased under LD conditions. Interestingly, the enhancement of light-induced stomatal opening and increased SOC1 expression in guard cells due to LD conditions persisted for at least 1 week after plants were transferred to SD conditions. We then investigated histone modification using chromatin immunoprecipitation-PCR, and observed increased trimethylation of lysine 4 on histone 3 (H3K4) around SOC1. We also found that LD-dependent enhancement of light-induced stomatal opening and H3K4 trimethylation in SOC1 were suppressed in the ft-2 mutant. These results indicate that photoperiod is an important environmental cue regulating stomatal opening, and that LD conditions enhance light-induced stomatal opening and epigenetic modification (H3K4 trimethylation) around SOC1, a positive regulator of stomatal opening, in an FT-dependent manner. Thus, this study provides novel insights into stomatal responses to photoperiod.