Objectives Hospital emergency departments (ED) are often overcrowded, and patients using ED for mental health (MH) reasons contribute in great part to this situation. In Quebec, in 2014-15, 17% of ED users with mental disorders had visited ED at least 4 times for various reasons. These patients' frequent ED use usually reflects the inadequacy of the services provided to them. A better understanding of the underlying reasons behind this frequent ED use would enable stakeholders to formulate recommendations that would help improve services, making them more suited to the needs of these patients. The aim of this study was to identify the factors that encourage or limit the use of ED by frequent ED users, the term "frequent ED users" being defined as patients who use ED at least 3 times over a one-year period. Methodology Between April and September 2021, 20 ED professionals were interviewed concerning factors they perceived as encouraging or limiting ED use among frequent ED users. Participants worked in a psychiatric ED or in other hospital services (e.g., assessment-liaison module), or were partners of the ED within the territory (e.g., crisis centers). Study data were analyzed using a content analysis method carried out in various stages (e.g., data transcription, content coding), and guided by a conceptual framework comprised of 4 categories of factors that encourage or limit frequent use for MH reasons. These factors were related to the healthcare system, patient profiles, health professionals, and the MH network's organizational characteristics. Results More encouraging factors than limiting ones were identified as pertains to ED use. Most factors were associated with the healthcare system (and particularly with the unavailability of MH services), and with patient profiles, more specifically those with complex mental disorders compounded by psychosocial problems. Organizational characteristics, in particular the deployment of innovations in the ED or in partnership with it, although not widely deployed overall, tended to limit ED use. Conclusion This study highlights the importance of developing more innovations in the ED and in conjunction with other hospital and community services to better meet the needs of frequent ED users, and thus reduce their use of these services. ED should optimize their role in the screening, brief treatment, referral, and quality monitoring of services for patients, particularly those not adequately served by outpatient MH services.