This study investigated the psychological, sociocultural, and normative determinants of spontaneous prosocial behavior, operationalized as the intention to return a found sealed envelope containing identifiable contents to its presumed owner. Drawing on a cross-cultural sample of 412 adult participants recruited across five culturally distinct national contexts Uganda, Japan, Germany, Brazil, and Canada — the study employed a quantitative survey design to examine how altruistic motivation, civic norm adherence, trust in strangers, cultural collectivism, perceived social obligation, and prior prosocial experience predict envelope return intention. Univariate analyses revealed that altruistic motivation (M = 3.91, SD = 0.81) and civic norm adherence (M = 3.78, SD = 0.74) were the most consistently endorsed psychosocial constructs across the sample, while overall return intention was affirmed by 67% of respondents. Bivariate correlation analyses demonstrated that all six predictor variables were significantly and positively correlated with return intention, with altruistic motivation emerging as the strongest bivariate correlate (r = 0.57, p < .001). Structural equation modelling (SEM) confirmed these relationships in a multivariate framework, with altruistic motivation (β = 0.312), civic norm adherence (β = 0.274), and perceived social obligation (β = 0.221) emerging as the three strongest direct predictors of return intention. Indirect pathways further revealed that cultural collectivism influenced return intention through its effects on civic norm adherence and trust in strangers. The model demonstrated excellent fit (CFI = 0.961; RMSEA = 0.047; SRMR = 0.051). Findings underscore the universality and cross-cultural variability of altruistic norms, suggesting that prosocial behavior, while context-sensitive, is meaningfully anchored in shared human motivational and normative structures. Recommendations are offered for public behavior campaigns, educational policy, and institutional trust-building initiatives.