The ongoing debate over the positive effects of a growth mindset on student outcomes has called for investigations into its contextual moderators (see Yeager & Dweck in Am Psychol 75(9):1269--1284, 2020. 10.1037/amp0000794). This study examined the potential moderating role of parental support in the associations of a growth mindset with student achievement (reading, math, science) and persistence. We used linear mixed effects analyses on the PISA 2018 dataset consisting of 468,059 15-year-old students from 76 countries/regions, accounting for the nested structure at the country level, measurement errors, sampling weights, and all plausible values for achievement. After controlling for student gender, grade, and socioeconomic status, findings show that both growth mindset and parental support positively predicted all achievement outcomes. Importantly, results show significant interactions between growth mindset and parental support for all four outcomes, suggesting that the growth mindset was more positively linked to achievement and persistence when students perceived higher parental support. The strength of the growth mindset x parental support interaction varied across countries/regions, with stronger effects found in cultures with higher collectivism. These findings highlight how parental support strengthens the link between growth mindset and student achievement and persistence, with variations across cultures. Theoretical and practical implications are discussed.