Background Many studies have revealed the positive impact of mindfulness training on mental health and proposed equanimity as a general outcome in contemplative research. Despite recent interest, relatively few studies have empirically examined equanimity and measurement instruments are still lacking. The main goal of this study was to develop an Equanimity Scale (the EQUA-S) in a Western population with or without meditation experience, based on previous definitions of equanimity, in order to investigate its relations with the relevant psychological constructs and health outcomes. Methods Adults from the general population ( N = 265; M age = 34.81) completed various measures: the EQUA-S, mindfulness, hyper-sensitivity, avoidance and fusion, impulsivity, personality, alexithymia, sensitivity to punishment and reward and frequency of problematic addictive behaviors. The dimensionality of the EQUA-S was examined using Factor Analyses. The convergent validity of this new scale was investigated using Pearson’s Correlations. Results The results of a factor analysis revealed two dimensions of equanimity: an even-minded state of mind (E-MSM) and a hedonic independence (HI) component. While the E-MSM was positively related to emotional stability, adaptive emotional regulation and several mindfulness-related abilities, HI was found to correlate negatively with addictive issues. Discussion The relations with personality constructs and possible related cognitive processes are discussed.