Background: The nature, incidence, and severity of adverse events (AEs) in hospital emergency departments (EDs) pose significant obstacles to patient safety and the quality of care provided globally and within Nigerian healthcare systems. While reporting of AEs may capture some incidents, healthcare professionals’ experiences regarding the types, the frequency of triggers, and detection measures remain understudied. Purpose: This study aimed to evaluate the knowledge and experiences of nurses and doctors regarding AEs in the EDs of three selected teaching hospitals in Southwestern Nigeria.Methods: An explanatory sequential mixed-methods design was used, involving 64 nurses and 57 doctors working in the emergency units of the selected hospitals. Data were collected using a 57-item structured, validated patient safety incident (PSI) questionnaire and six focus group discussions (FGDs) with 36 healthcare professionals purposively selected from the quantitative participants. Quantitative data were analyzed using descriptive statistics and the chi-square test, while qualitative data were thematically analyzed using ATLAS software.Results: Quantitative findings revealed that 56.2% of healthcare professionals possessed good knowledge of commonly occurring AEs; however, 65.3% had limited knowledge of specific triggers that cause AEs. Reporting methods mainly included forms and patient record reviews. Qualitative findings provided deeper insights into these results and revealed four key themes: experiences of AEs in emergency care, patterns and types, reporting practices and barriers, and suggestions for improved reporting. Participants reported equipment malfunctions and adverse drug reactions (ADRs) as frequently experienced AEs, and cited severe workloads, time constraints, the absence of standardized reporting protocols, fear of punishment, and restricted access to digital reporting tools as challenges to reporting.Conclusion: Despite good awareness demonstrated by healthcare professionals, significant organizational and system-level barriers hinder effective AE detection and reporting. Strengthening training on AE identification, introducing standardized digital reporting protocols, and promoting a non-punitive reporting culture may improve patient safety in EDs.