Body image flexibility shows promise for understanding positive body image in young people. However, its adaptive functions remain unclear due to the lack of youth-oriented models and existing focus on inflexibility in unidimensional measures. This study developed a measure of body image flexibility and inflexibility for youth by adapting the Multidimensional Psychological Flexibility Inventory (MPFI) with 724 male and female Australian high school and university students aged 11 to 30 years. The Body Image Flexibility and Inflexibility Scale (BIFIS) replicated the higher-order MPFI structure (flexibility and inflexibility), with modifications required to lower-order factors to achieve acceptable fit. The final model (24 items; two higher-order and eight lower-order factors) was invariant across gender and age cohorts. The higher-order body image flexibility and inflexibility factors were internally consistent, stable over one month, correlated in expected directions with adaptive and maladaptive body image and eating constructs and well-being, and explained adaptive outcomes beyond measures of body satisfaction and unidimensional body image flexibility. The BIFIS provides a way to measure how young people respond to body image threats and expands existing conceptual models by including adaptive and maladaptive processes. Future work could refine the items, further validate the factor structure, and consider more experiential measurement formats.