The Group Cohesiveness Scale (GSC) is a newly composed scale which combines items drawn from the Therapeutic Factor Inventory of Cohesiveness and the Engaged Subscale of the Group Climate Questionnaire -Short Form, based on previous research findings regarding the behavioural function of cohesion. This study aims to examine the psychometric properties of this new scale. A seven-item GCS questionnaire was completed along with a UCLA Loneliness Scale and Cohesion to Therapist Scale questionnaire for 135 inpatients aged between 22 and 64 who had undergone group psychotherapy sessions at the inpatients’ unit of a university hospital. The scale showed a high level of internal consistency. The level of correlation between the GCS and both the UCLA Loneliness Scale and Cohesion to Therapist Scale revealed the convergent and discriminant validity of the scale. Exploratory and confirmatory factor analysis supported a uni-dimensional structure for the measurements taken. The GCS is a reliable and valid measure of cohesiveness, and its brevity will support assessment of the group psychotherapy process, especially in psychiatric inpatient units.