The primary objective of this paper is to develop and validate an instrument for massively multiplayer online game (MMOG) addiction. Three stages of instrument development were undertaken to achieve this objective: item generation, scale development, and instrument testing. The results of a confirmation factor analysis confirmed the existence of seven dimensions of MMOG addiction, namely Salience, Mood Modification, Tolerance, Withdrawal, Conflicts, Relapse, and Loss of Control. The final instrument exhibits adequate levels of reliability, convergent validity, discriminant validity, and nomological validity. The instrument is expected to assist future empirical research on MMOG addiction. The theoretical and practical implications are discussed.