This study investigates how the implementation of a social login feature on review platforms affects users’ emotional expressions. It further examines whether this effect varies by national culture, particularly along the dimension of individualism versus collectivism. Drawing on impression management theory, we employ a natural experiment and multilevel regression analysis, utilizing 58,459 TripAdvisor reviews from 140 countries. Our results demonstrate that social login increases positive emotional expressions and decreases negative ones. Importantly, individualism moderates this effect only for negative emotions, suggesting that cultural norms more strongly regulate negative expression than positive sentiment. The absence of moderation for positive emotions implies more universal norms for positivity in socially visible online contexts. These findings highlight that emotional expressions are shaped by the interplay between platform features and cultural contexts, with emotional valence playing a key role. The study underscores the need for culturally sensitive and emotionally aware platform design.