Objective Current diagnostic criteria for somatoform disorders demand revisions due to their insufficient clinical as well as scientific usability. Various psychological and behavioral characteristics have been considered for the proposed new category Somatic Symptom Disorder (SSD). With this study, we were able to jointly assess the validity of these variables in an inpatient sample. Methods Using a cross-sectional design, we investigated N=456 patients suffering from somatoform disorder, anxiety, or depression. Within one week after admission to the hospital, informed consent was obtained and afterwards, a diagnostic interview and a battery of self-report questionnaires were administered. Logistic regression analyses were performed to determine which variables significantly add to construct and descriptive validity. Results Several features, such as somatic symptom severity, health worries, health habits, a self-concept of being weak, and symptom attribution, predicted physical health status in somatization. Overall, our model explained about 50% of the total variance. Furthermore, in comparison with anxious and depressed patients, health anxiety, body scanning, and a self-concept of bodily weakness were specific for DSM-IV somatoform disorders and the proposed SSD. Conclusions The present study supports the inclusion of psychological and behavioral characteristics in the DSM-5 diagnostic criteria for somatoform disorders. Based on our results, we make suggestions for a slight modification of criterion B to enhance construct validity of the Somatic Symptom Disorder.