Thin ideal internalization is widely implicated in women's body image and eating disturbances. A recently proposed multidimensional operationalization of internalization suggests the brevity and construct validity of existing questionnaires may limit the assessment of thin ideal internalization. Therefore, this research aimed to develop a new questionnaire (i.e., Thin Ideal Internalization Assessment; THIINA) to comprehensively assess thin ideal internalization. In Study 1, 301 female participants were administered the THIINA. Exploratory factor analyses revealed the 17-item THIINA had a stable 3-factor structure reflecting thin idealization, thin overvaluation, and thin behavioral drive. In Study 2, 337 female participants were administered the THIINA and validation measures. Confirmatory factor analysis confirmed stability of the 3-factor structure and findings supported convergent, discriminant, and incremental validity of the THIINA. Support for temporal stability was found within a sub-sample of participants (n = 132). The THIINA demonstrated strong psychometric properties, a stable three-factor structure representing theoretically-driven domains, and support for the creation of a composite score representing overall thin ideal internalization. These findings suggest the multidimensional operationalization and measurement of thin ideal internalization could improve theoretical and clinical understanding of the impact of thin ideal internalization on women's body image and eating.