The purpose of this study was three-fold: first, it aimed to develop a scale to shed light on English as Foreign Language (EFL) teachers' perceptions of using Mobile Social Network Sites (MSNSs) in language teaching and learning. To this end, a 36-item MSNSs Utility Perceptions Inventory (MUPI) was developed. Second, the current study made an attempt to investigate the construct validity and reliability of the MUPI through collecting data from 184 Iranian EFL teachers. The factorability of the data has been substantiated by running Kaiser-Meyer-Olkin test of Sampling Adequacy (KMO) and Bartlett's Test of Sphericity. The results of Exploratory Factor Analysis (EFA) revealed a three-factor solution for the inventory items. Furthermore, it was deemed that the scale had a high level of internal consistency. Third, this study strove to explore EFL teachers' perceptions of MSNSs use by analyzing the results of the administered scale. The findings showed that teachers put more emphasis on factor one (Advantages of MSNSs for Teaching and Learning) and factor three (Barriers to Using MSNSs for Teaching and Learning). The findings also indicated that approximately half of the teachers believed MSNSs allow them to have a more detailed, in depth conversation with their peers and colleagues in the course. Moreover, student distraction, insufficient Internet bandwidth/speed, and insufficient technical support for teachers were some barriers to using MSNSs in classrooms. It was also confirmed that developing communication skills, increasing communication, motivating students, and sharing feedback are some advantages of using MSNSs for teaching and learning. This study has some implications for EFL teachers, teacher educators, and syllabus designers to pay particular attention to the positive features of MSNSs and harness their pedagogical benefits in English language teaching and learning on the one hand, and teacher education courses on the other hand.