["Journal of Personality, EarlyView. ", "\nABSTRACT\n\nObjective\nThis study examined the intra‐individual associations between prosocial behavior and two dimensions of subjective well‐being—life satisfaction and hedonic balance—in the daily lives of young adults.\n\n\nMethod\nTwo samples of Italian and Spanish participants aged 18–35 completed self‐report measures at different intervals: a daily sample assessed for ten consecutive days (N = 388; 76% women) and a weekly sample assessed for five consecutive weeks (N = 260; 80.3% women). The weekly interval was included as a “sensitivity analysis” to evaluate the stability of effects over longer periods. We employed Random Intercept Cross‐Lagged Panel Models (RI‐CLPMs) to investigate within‐person dynamics while accounting for stable between‐person differences.\n\n\nResults\nIn the daily sample, prosocial behavior was associated with increases in life satisfaction from one day to the next one. This effect was not observed across weekly assessments. Although we did not find any empirical evidence that prosocial behavior affects hedonic balance, within‐person correlations between variables were observed in the daily sample, but these tended to disappear in the weekly period.\n\n\nConclusions\nIncorporating prosocial behaviors into daily routines may promote young adults' life satisfaction. This study contributes to the growing knowledge on how prosocial behavior influences subjective well‐being in everyday life.\n\n"]