Aims A previous study has reported a significant month-of-birth effect on mood seasonality in the northern hemisphere. Higher mood seasonality was observed for university students born during spring or summer months (long photoperiod) compared to those born during autumn or winter months (short photoperiod). The aim of this study was to test the hypothesized photoperiod effect by comparing the correlation between month of birth and mood seasonality in two countries located at the opposite poles of the terrestrial globe: Italy (northern hemisphere) and New Zealand (southern hemisphere). On the basis of the photoperiod-at-birth hypothesis, we expected to find higher mood seasonality among individuals born in months with longer photoperiods in both countries. Methods The Seasonal Pattern Assessment Questionnaire was administered to 1514 young adults (1088 women, 426 men; 1027 Italians, 487 New Zealanders), with ages ranging from 18 to 34 years. The Global Seasonality Score, which is a measure of mood seasonality, was calculated from the Seasonal Pattern Assessment Questionnaire. Results A significant overall month-of-birth effect was observed on the Global Seasonality Score, but only for men. Men born in April and August (corresponding in Italy to a long photoperiod, in New Zealand to a short photoperiod) had higher mood seasonality than those born in February, regardless of country. Conclusions A significant month-of-birth effect was found on mood seasonality, but results do not support the hypothesis based on the photoperiod effect.