Healthcare professionals frequently encounter various forms of aggression, ranging from verbal abuse to physical assaults, which can compromise both their occupational well-being and patient-care quality. Despite its prevalence and serious consequences, workplace aggression is often underreported due to a lack of standardized assessment tools. This study aims to develop a valid Italian version of the Hospital Aggressive Behaviour Scale-Users. The scale's structure was evaluated using exploratory (EFA) and confirmatory (CFA) factor analyses on two samples of healthcare professionals during and after the pandemic. Reliability, measurement invariance, and nomological validity were examined. EFA revealed a two-factor structure comprising eight items (χ2 = 59.651, df = 13, p = 0.00; CFI = 0.98; TLI = 0.95; RMSEA = 0.07; SRMR = 0.02), distinguishing non-physical and physical aggression, and meeting all recommended criteria. CFA confirmed this structure, demonstrating good reliability and outperforming alternative models. The same factor structure was confirmed in standard (χ2 = 35.01, df = 19, p = 0.00; CFI = 0.99; TLI = 0.99; RMSEA = 0.03; SRMR = 0.02) and emergency (χ2 = 30.65, df = 19, p = 0.04; CFI = 0.98; TLI = 0.97; RMSEA = 0.06; SRMR = 0.04) contexts. Full residual invariance was found across job tenure groups. Aggression was positively associated with emotional exhaustion, psychological distance, psychosomatic symptoms, post-traumatic stress symptoms, and turnover intentions while negatively related to job satisfaction. Nurses and healthcare assistants reported higher levels of aggression than doctors. This study provides a reliable, context-specific instrument for documenting and analysing outsider aggression. The insights can inform targeted interventions, contributing to a healthier hospital environment.