Implicit theory has been relatively well-studied in the areas of intelligence and personality but remains less investigated in mental health. This article aims to analyze the psychometric properties of the Implicit Thoughts, Emotion, and Behavior Questionnaire (ITEB-Q; Schleider & Weisz, 2016a). We tested its factorial validity, measurement invariance across gender and two racial groups, as well as criterion validity in a large, diverse sample of adolescents. The sample consists of 817 adolescents aged between 14 and 19 and in 9th-12th grade from a high school in a Southern city in the United States. The confirmatory factorial analysis results corroborate the three-factor structures (thoughts, emotion, behavior) of the ITEB-Q. Multigroup analyses established the scalar invariance across gender as well as across Black and White adolescents. Further, latent mean comparison showed that female adolescents reported lower means on the emotion and thought subscales than male counterparts, and Black adolescents reported higher means across three subscales than White adolescents. The ITEB-Q and its subscales demonstrate good internal reliability and concurrent validity with measures of life satisfaction, resilience, emotional regulation, and emotional problems. Overall, our findings suggest that the ITEB-Q produces valid and reliable scores to assess implicit theory of thoughts, emotion, and behavior among adolescents.