Background Bisexual people are at an elevated risk for non-suicidal self-injury (NSSI). Psychological factors including self-esteem and thwarted belongingness may help explain this risk. The aim of the current study was to investigate associations between self-esteem, thwarted belongingness and NSSI urges and behavior in young bisexual people. Methods Participants aged 16–25 from 25 countries took part in this microlongitudinal online survey study (N = 207). Mixed-effects linear and logistic regression were used for analysis, given the nested structure of data. Analysis examined the relationship between self-esteem and thwarted belongingness on NSSI urges and behavior at the same point in time, and lagged by one week. Results At the same timepoint, self-esteem and thwarted belongingness both had significant between- and within-person associations with NSSI urges and behavior. For lagged models, self-esteem had significant between-person effects on urges, and thwarted belongingness had significant within-person effects. For NSSI behavior, both variables were only associated with between-person effects. Conclusion Findings support previous research suggesting the importance of self-esteem in explaining NSSI among sexual minorities. Preventative and intervention strategies to improve self-esteem may help reduce NSSI risk. Future studies should focus upon the experiences of bisexual people with intersecting identities and ensure that studies are statistically powered from inception to detect effects.