A sociocultural motives perspective (SMP) on Big Five relationships is introduced. According to the SMP, Agreeableness and Conscientiousness elicit assimilation to sociocultural norms, Openness elicits contrast from these norms, and Extraversion and Neuroticism are independent of sociocultural assimilation and contrast. Due to sociocultural assimilation, then, relationships of Agreeableness and Conscientiousness with an outcome wax (become more positive or less negative) with that outcome's increasing sociocultural normativeness. Due to sociocultural contrast, relationships of Openness with an outcome wane (become less positive or more negative) with that outcome's increasing sociocultural normativeness. We tested the SMP using religiosity as our outcome. Study 1 included 4 cross-sectional self-report data sets across 66 countries (N = 1,129,334), 50 U.S. states (N = 1,057,342), 15 German federal states (N = 20,885), and 121 British urban areas (N = 386,315). Study 2 utilized informant-report data across 37 countries (N = 544,512). Study 3 used longitudinal data across 15 German federal states (N = 14,858). Results consistently supported the SMP. Relationships of Agreeableness and Conscientiousness with religiosity were more positive in religious sociocultural contexts than in secular contexts. Relationships of Openness with religiosity were more negative in religious sociocultural contexts than in secular contexts. At a more general level, the SMP offers theory-driven explanations for cross-cultural variations in Big Five relationships with their outcomes.