A large body of research has examined the relationship between belief in a just world (BJW) and well-being. However, this research, and work on BJW more broadly, has predominantly employed experimental and cross-sectional methods, which may not adequately capture how BJW functions in daily life. To help address this, we considered how two forms of BJW-believing the world is just for the self (personal-BJW) and just for others (general-BJW)-relate to various aspects of well-being between persons in a cross-sectional study (N = 512) and, critically, within persons in a 2-week naturalistic daily diary study (N = 132; 1439 daily reports). Results revealed that both personal- and general-BJW varied between- and within-individuals. Moreover, personal-BJW was not only more robustly related to greater well-being than general-BJW at the between-person level, consistent with prior work, but also at the within-person level. Overall, our diary findings suggest that BJW fluctuates in daily life and that these fluctuations covary positively with well-being.