A self-report scale to measure adolescents’ involvement in parental figures’ conflicts, disagreements, or relationships, a phenomenon known as triangulation, was developed, and initial evidence for reliability and validity was obtained using a three-study format. In Study 1, the 24-item Triangular Relationship Inventory (TRI) was developed using classical test theory in a sample of college-aged students and found to have a multidimensional factor structure. In Study 2, the multidimensional factor structure was replicated in a new sample of college-aged students. Evidence of reliability was supported with excellent internal consistency. Construct validity was supported by replicating the multidimensional factor structure of the TRI. A strong correlation between the TRI and the Intergenerational Triangulation subscale of the Personal Authority in the Family System Questionnaire indicated criterion-related validity. Convergent validity of the TRI was indicated by a negative correlation between the TRI and the Differentiation of Self Inventory–Short Form, a positive correlation between the TRI and the Hopkins Symptom Checklist-21, and a positive correlation between the TRI and the State–Trait Inventory for Cognitive and Somatic Anxiety. Discriminant validity was evidenced by a low correlation between the TRI and the Satisfaction With Life Scale. Study 3 found the test–retest reliability of the TRI over a 4-week period to be strong, at .84. In summary, the TRI appears to have good psychometric properties and contributes to the literature in family systems theory by enabling a more nuanced measurement of triangulation than has been previously available.