Background At the height of the COVID-19 pandemic, several mental health care providers were obliged to shut down outpatient services, including group therapy and psychoeducational sessions. The lockdown in many countries is a serious threat to people’s mental well-being, especially for individuals with severe mental illnesses. Discontinued outpatient treatments and disruption of daily routines are considered to be risk factors for destabilization of patients with mental illness. Objective The aim of this study was to evaluate the acceptability, usability, and feasibility of a group chat program to replace cancelled face-to-face group sessions in an outpatient psychiatric department. Methods Participants (N=33) were recruited in the outpatient department of a large university medical center in Leipzig, Germany. Former face-to-face group participants were invited to take part in a therapist-guided group-chat for 4 weeks (8 sessions) and were asked to evaluate the program via self-administered standardized questionnaires at baseline (T0, preintervention), after every chat session (T1), and posttreatment (T2, after 4-6 weeks). The chat groups were specific to the following mental disorder diagnoses and based on the same therapeutic principles and techniques as the former face-to-face groups: anxiety, depression, obsessive-compulsive disorder, and adult attention-deficit/hyperactivity disorder (ADHD). Sociodemographic measures, attitudes toward the COVID-19 pandemic, depressive symptoms (Patient Health Questionnaire-9), quality of life (abbreviated World Health Organization Quality of Life assessment), treatment credibility/expectancy (Credibility Expectancy Questionnaire), and participants’ satisfaction (Client Satisfaction Questionnaire-8 [ZUF-8]) were measured. Results Participants joined an average of 5 out of 8 offered chat sessions. Participation rates in the respective groups were highest in the ADHD group (8.6/11, 78%) and lowest in the anxiety group (3.7/9, 41%). The overall preintervention level of depressive symptoms was moderate and showed a slight, nonsignificant improvement at posttreatment (T0: mean 10.7, SD 5.5; T2: mean 10.2, SD 5.5). A similar result was observed regarding quality of life (T0: median 41.7-68.8; T2: median 50-70.3). Treatment credibility and expectancy scores were medium-high (T0: meancredibility 18.1, SD 3.8; meanexpectancy 11.2, SD 5.1; T2: meancredibility 17.1, SD 4.8; meanexpectancy 10.3, SD 5.8). Further, significant correlations were detected between posttreatment expectancy score and posttreatment PHQ-9 score (r=–0.41, P=.02), posttreatment physical quality of life (r=0.54, P=.001), and posttreatment psychological quality of life (r=0.53, P=.002). Overall, participants’ satisfaction with the program was very high, both after chat sessions and at posttreatment (ZUF-8: mean score 20.6, SD 1.0). Of all participants, a majority (27/31, 87%) rated the program as excellent/good and indicated they would recommend the group chat program to a friend in need of similar help (23/31, 74%). Conclusions A therapist-guided group chat program to substitute outpatient group setting treatment during the COVID-19 lockdown was shown to be feasible, usable, and highly acceptable for participants. Web-based programs such as this one provide an easy-to-implement tool to successfully stabilize participants during a difficult time, such as the COVID-19 pandemic. Trial Registration German Clinical Trials Register DRKS00021527; https://tinyurl.com/3btyxc2r