Although social scientists have studied group-based relative deprivation (GRD) for more than half a century, there is no established, validated scale for measuring GRD. We developed the GRD scale, which can be administered in different intergroup contexts, and examined its psychometric properties in two samples (ns = 334 & 309), its construct validity in two experimental studies (ns = 98 & 271), and its convergent validity in a correlational study (n = 189). Analyses based on Classical Test Theory showed that the scale has high internal consistency, converging on similar estimates across several different internal consistency coefficients (⍺, ⍵_t, ⍵_h), and demonstrating excellent goodness-of-fit indices. Using Item-Response Theory approaches, Mokken scaling analysis showed that the scale is unidimensional and strong. The scale also differentiated between relevant groups (GRD scores were higher among women than men). Importantly, we demonstrated the construct validity of the scale (as conceptualized by Borsboom et al., 2004) by showing that objective group-based disadvantage in tangible, monetary terms is causally related to scores on the GRD scale. Finally, we demonstrated the scale’s convergent validity.