Objective: This study aims to determine the effect of assertiveness training on teamwork attitudes toward the crisis experienced by a standardized patient during epileptic seizures and on the clinical skills of nursing students. Methods: This randomized controlled study was conducted following a pre-test–posttest design with an experimental group (n = 15) and a control group (n=14). Data were collected using a personal information form, the Clinical Skills Assessment Form, the Rathus Assertiveness Inventory, and the TeamSTEPPS®-Teamwork Attitude Questionnaire. The study was conducted in 5 stages. In the first stage before training, the personal information form as well as the Rathus Assertiveness Inventory and the TeamSTEPPS®-Teamwork Attitude Questionnaire were administered to both groups. In the second stage, assertiveness training was provided to the experimental group, and in the third stage, training on the management of epileptic seizures was provided to both groups. In the fourth stage, a simulation was done, and the groups’ performances were evaluated using the Clinical Skills Assessment Form. Finally, in the last stage, the Rathus Assertiveness Inventory and the TeamSTEPPS®-Teamwork Attitude Questionnaire were re-administered to all groups. Results: The comparison of the experimental and control groups’ pre-test–posttest scores on the Rathus Assertiveness Inventory, the TeamSTEPPS®-Teamwork Attitude Questionnaire, and the subscales of TeamSTEPPS®-Teamwork Attitude Questionnaire indicated that the leadership posttest scores and the Clinical Skills Assessment Form total scores of the experimental group were significantly higher (P <.05). The experimental group’s Rathus Assertiveness Inventory posttest scores were significantly higher than their pre-test scores (P <.05). Conclusion: It can be concluded that assertiveness training was effective in improving teamwork attitudes, leadership role, and clinical skills based on simulations.