Previous research indicates that trainees often withhold important information related to their clients’ progress and their reactions to supervision. Moreover, factors associated with the occurrence of supervisee nondisclosure (SND) are not well established. As one of the few studies to compare clinically related with supervision-related nondisclosure, we tested the relation of these two dimensions of the construct, as measured by Siembor and Ellis’s (2012) Supervisee Nondisclosure Scales, to three important process variables: perceptions of (a) the supervisory alliance, as measured by Bahrick’s (1989) Working Alliance Inventory-Trainee; (b) collaborative supervision, as measured by Rousmaniere and Ellis’s (2013) Collaborative Supervision Behavior Scale; and (c) explicitly relational supervisor behaviors, as measured by Shaffer and Friedlander’s (2017) Relational Behavior Scale. Participants were 257 North American trainees, predominately White female doctoral students receiving supervision in a practicum or internship setting. As hypothesized, the three process variables inversely contributed to the multivariate composite of SND, jointly accounting for roughly one quarter of the variability; alliance and relational behavior were uniquely significant contributors (24.5% and 7%, respectively). Follow-up analyses indicated that only alliance perceptions contributed unique variance (23%) to supervision-related nondisclosures, which were endorsed much more frequently than clinically related nondisclosures in this sample.