Background: The present study considers a measure of positive body image, the Functionality Appreciation Scale (FAS), which assesses one’s perception of appreciating, respecting, and honouring the body for what it can do [3]. Differential functioning of the scale across groups (i.e., gender) is yet to be investigated. The present study contributes to this area of knowledge via the employment of Item Response Theory (IRT) analyses. Methods: A sample of 386 adults from Canada, Australia, New Zealand, Ireland, the United Kingdom (UK), and United States of America (USA) were assessed online (N = 394, 54.8% men, 43.1% women, Mage = 27.48, SD = 5.57).Results: The two-parameter logistic model employed to observe IRT properties indicated that all items demonstrated, although variable, strong discrimination capacity. Considering the DIF across men and women, all items demonstrated psychometric invariance across groups indicating that FAS measures FA equally in men and women.Conclusions: The items showed increased reliability for latent levels of ± 2 SD from the mean level of Functionality Appreciation. The implications and interpretations of the findings for clinical practice are discussed.