First responders and other public safety personnel (PSP) are at elevated risk of experiencing potentially psychologically traumatic events, mental disorders, and barriers to accessing mental healthcare. Internet-delivered cognitive behavioural therapy (ICBT) is an effective and accessible treatment for various common mental disorders. Therapistguided ICBT is more effective than self-guided ICBT, but self-guided ICBT is easier to implement on a large scale. Designing self-guided ICBT to be more engaging for users has been recommended to improve engagement and outcomes. One way to engage users is to provide them with access to an online discussion forum. Self-guided ICBT interventions that have included online discussion forums have demonstrated excellent treatment outcomes, comparable to those of guided ICBT, but there is a need for research experimentally testing the impact of adding online discussion forums. The current study objectives were to evaluate a transdiagnostic, self-guided ICBT program tailored for PSP, explore whether a therapist-moderated online discussion forum impacts outcomes, and analyze participant feedback to inform future efforts to implement forums in ICBT. The current study consisted of a randomized trial (n = 107) with two conditions, both of which involved an 8-week, transdiagnostic self-guided ICBT course tailored for Canadian PSP. In one condition, but not the other, the ICBT course included a therapistmoderated online discussion forum. We administered several self-report measures during eligibility screening and at 8 and 20 weeks post-enrollment. Primary outcomes included changes in symptoms of major depressive disorder, generalized anxiety disorder, and posttraumatic stress disorder. Secondary outcomes included treatment satisfaction, treatment engagement, patient-program alliance, and changes in health service use, resilience, and subjective wellbeing. We compared treatment outcomes across the two conditions using multilevel modeling. We used qualitative analyses to further examine written comments related to treatment satisfaction and participants’ experiences with the online discussion forum. Across conditions, most participants accessed at least four of the five lessons of the ICBT course (n = 59, 55.1%) and completed post-treatment questionnaires at 8 weeks (n = 83, 77.5%) and 20 weeks post-enrollment (n = 73, 68.2%). Participants in both conditions who reported clinically significant symptoms at pre-treatment reported large and statistically significant reductions in symptoms (ps < .05, ds > 0.97). Participants also evidenced good treatment satisfaction and engagement. There was no evidence of group differences on any treatment outcomes, and most outcomes were comparable to those of therapist-guided ICBT tailored for PSP. However, 30.0% of prospective participants in the present study either failed to access the ICBT course or withdrew from the study, compared with only 5.7% for therapistguided ICBT. Participants created only nine posts in the online discussion forum. Qualitative analyses revealed that the forum had both strengths (e.g., it helped some participants feel less alone) and limitations (e.g., some participants disliked the relatively low level of activity). The current results support continued research on and implementation of self-guided ICBT for PSP and other groups. Results also suggest a need for further research to determine how online discussion forums should be structured and implemented in self-guided ICBT and to clarify their impact on outcomes.