The concept of negative partisanship has lately become a highly salient topic, yet its current measurements are far from optimal as they do not account for negative partisanship’s nature as a social identity, nor are they applicable to multiparty systems. In this paper, we validate the negative partisanship (NPS) scale. By relying on expert interviews, cognitive pre-tests and a large-N survey in two countries, Germany (N = 1,911) and Italy (N = 1,440), we provide a construct validation using a nomological network based on previous studies and social identity literature. Our results show the applicability of the new instrument, the full five-item version as well as shorter conceptualizations, for the measurement of negative partisanship in multiparty systems.