Decentering, a detached, observer perspective on one’s mental activity, is an important concept for understanding mental health. Meta-awareness, people’s awareness of their own current mental activity, is thought to facilitate decentering. However, trait measures of these concepts are not available or have validity concerns. We sought to create a theoretically derived measure of meta-awareness and decentering that allowed an exploration of questions in the literature regarding whether there are multiple forms of decentered awareness and whether meta-awareness and external awareness are distinct. Across six samples and 2,480 participants, we developed the 25-item Multidimensional Awareness Scale, with subscales assessing meta-awareness (present moment awareness of mental activity), decentered awareness (meta-awareness from a psychologically distant perspective), and external awareness (present moment awareness of the world outside of oneself). The scales demonstrated acceptable reliability and validity. Results are discussed in terms of the conceptual implications of the scale structure and its potential uses.