Public safety personnel (PSP) face high rates of mental health problems and many barriers to care. Initial outcomes of transdiagnostic internet-delivered cognitive behavioural therapy (ICBT) tailored for PSP are promising, but prior research has not evaluated outcomes of PTSD-specific ICBT among PSP or PSP's preferences for transdiagnostic or PTSD-specific ICBT. The current paper presents the initial outcomes (N = 150) of a mixed-methods observational study designed to (a) investigate preferences for transdiagnostic or PTSD-specific ICBT among PSP with elevated symptoms of PTSD and/or a primary concern with PTSD symptoms and (b) explore potential differences in client engagement, satisfaction, and symptom changes between the two forms of ICBT. PSP completed questionnaires before and after their preferred ICBT program. Mixed-methods analyses included generalized estimating equations, descriptive statistics, and inductive conventional qualitative content analysis. More clients (n = 85; 57 %) selected transdiagnostic ICBT than PTSD-specific ICBT (n = 65; 43 %), but the difference in the number of clients who selected each course was not statistically significant. Clients in both ICBT programs reported similar and favorable treatment satisfaction (e.g., 98 % would recommend the course to a friend), treatment engagement (i.e., 69 % accessed at least four of the five lessons), and pre-post improvement in symptoms (e.g., Hedges' g = 0.81 for reduction in PTSD symptom). Transdiagnostic ICBT resulted in greater reductions in symptoms of panic disorder than PTSD-specific ICBT. Qualitative analyses showed similarities across the ICBT programs in client feedback. The current study provides further evidence supporting the use and outcomes of ICBT for PSP in both transdiagnostic and disorder-specific formats. Implications for the literatures on PSP mental health and ICBT, as well as practical recommendations, are discussed.