Background: The patient's perception of external radiotherapy (RT) procedures and equipment is important to evaluate as a complement to endpoints such as treatment outcome and reproducibility. There is a lack of a proper, psychometrically robust instrument to evaluate the patient's comfort and experience of the external RT procedure. Hence, this study aimed to develop and test an instrument to measure the patient's experience during external RT. Material and Methods: A preliminary 34-item questionnaire was generated from research literature, expert consultations and patient interviews, and it was distributed to patients (n = 825) at 8 RT units in Sweden. The answers were subjected to item analysis and reduction by using exploratory factor analysis. The reliability of the final questionnaire was evaluated using Cronbach's alpha. Mean scale scores were compared across gender, length of RT and treatment area. Results: Most items were highly skewed towards positive responses. Scree plot analyses of the 34-item correlation matrix identified six underlying themes explaining 68% of the total variance. After item reduction, the 6 themes explained 73% of the variance in a 23-item questionnaire. Cronbach's alpha was satisfactory for all themes (between 0.79 and 0.9). Significant differences between treatment areas were found for two scales: situational unease and situational repose. Conclusion: The RT Experience Questionnaire is a tentatively valid and reliable instrument to measure how patients experience the external RT session process and the environment in the treatment room.