Community mental health centers (CMHCs) offer invaluable, publicly-funded treatment for serious mental illness (SMI). Unfortunately, evidence-based psychological treatments are often not delivered at CMHCs, in part due to implementation barriers, such as limited time, high caseloads, and complex clinical presentations. Transdiagnostic treatments may help address these barriers, because they allow providers to treat symptoms across multiple disorders concurrently. However, little research has investigated CMHC providers' experiences of delivering transdiagnostic treatments "on the ground," particularly for adults with SMI. Thus, the aim of the present study was to assess CMHC providers' perspectives on delivering a transdiagnostic treatment -- the Transdiagnostic Intervention for Sleep and Circadian Dysfunction (TranS-C) -- to adults diagnosed with SMI. In the context of a larger parent trial, providers were randomized to deliver a standard version of TranS-C (Standard TranS-C) or a version adapted to the CMHC context (Adapted TranS-C). Twenty-five providers from the parent trial participated in a semi-structured interview (n = 10 Standard TranS-C; n = 15 from Adapted TranS-C). Responses were deductively and inductively coded to identify themes related to Proctor's taxonomy of implementation outcomes. Four novel "transdiagnostic take homes" were identified: (1) transdiagnostic targets, such as sleep, can be perceived as motivating and appropriate when treating SMI, (2) strategies to bolster client motivation/adherence and address a wider range of symptom severity may improve transdiagnostic treatments, (3) balancing feasibility with offering in-depth resources is an important challenge for transdiagnostic treatment development, and (4) adapting transdiagnostic treatments to the CMHC context may improve provider perceptions of implementation outcomes.