The Interpersonal Behaviors Questionnaire (IBQ) in Sport assesses athlete perceptions of coach interpersonal behaviors that support or thwart athletes' basic psychological needs. While different sources of validity evidence for responses to the IBQ have been provided for adults, only validity evidence for relations to other variables has been provided with youth. Moreover, potential problems with internal structure for youths' responses to the IBQ have recently been noted and motivated the current study. Specifically, a three-phase mixed-methods design was employed to provide validity evidence for modification of the IBQ for youth. Phase I empirically examined the internal structure for youth athletes' responses to the IBQ. Phase II used cognitive think-aloud interviews to identify and then modify problematic IBQ items to create the Youth Interpersonal Behaviors Questionnaire in Sport (Y-IBQ). Phase III examined the psychometric properties of the Y-IBQ to provide initial validity evidence for this instrument in a youth athlete sample.