Mediated Social Touch (MST) technologies focus on enhancing a communication experience by sensing, transmitting, and simulating social touch between remote partners. With interest in developing MST technologies continuing to grow, it is important to create standardized methods for measuring the effect of these novel systems. We designed and validated a 9-item questionnaire to measure the "Social Disfordance" of Mediated Social Touch, with three scales that focus on Social Discomfort, Communicational Expressiveness, and Need for Additional Consideration. A high degree of "social disfordance" of an MST system signifies that it may not provide the appropriate social affordances for mediating touch in a particular context. The development of the Social Disfordance of Mediated Social Touch (SDMST) instrument included a systematic literature review, expert feedback, and think-out-loud piloting. Its refinement included an exploratory factor analysis with a subsequent reduction of questions and scales. We report its psychometric properties, including metrics of inter-item reliability, convergent validity, test-retest reliability, and concurrent validity, confirming that these properties are sufficient for future use. We conclude with examples of scoring, appropriate use, and a discussion of the limitations.