Assertive behavior is an essential professional competency that supports effective clinical communication, interprofessional collaboration, and patient safety. This study aimed to develop and examine a model of nurses’ assertive behavior based on emotional intelligence, organizational climate, and communication effectiveness in inpatient nursing services at BLUD RSUD dr. T.C. Hillers Maumere. A quantitative analytical observational study with a cross-sectional design was conducted among 120 staff nurses selected using proportionate stratified random sampling. Data were collected using validated and reliable questionnaires measuring emotional intelligence, organizational climate, communication effectiveness, and nurses’ assertive behavior. Data were analyzed using descriptive statistics and Structural Equation Modeling-Partial Least Squares (SEM-PLS) with SmartPLS version 4. Most respondents demonstrated high emotional intelligence (64.2%), perceived the organizational climate as good (60.0%), reported effective communication (68.3%), and demonstrated high assertive behavior (61.7%). SEM-PLS analysis showed that emotional intelligence had a positive and significant effect on assertive behavior (β = 0.352, p < 0.001), as did organizational climate (β = 0.287, p < 0.001) and communication effectiveness (β = 0.438, p < 0.001). Communication effectiveness was the strongest predictor. The model explained 71.4% of the variance in nurses’ assertive behavior (R² = 0.714). These findings indicate that strengthening emotional intelligence, organizational climate, and communication effectiveness may support nurses’ assertive behavior, professional communication, and patient safety.