Emotion dysregulation is associated with attention deficit/hyperactivity disorder (ADHD) and confers risk for behavior problems and functional impairment; however, there is little guidance on best practices for measurement in adolescents. We developed a parent-report version of the Difficulties in Emotion Regulation Scale (DERS-P)). Evidence of reliability and validity was evaluated in a large community online sample (Study 1: n = 978; M[subscript age] = 13.52 years; SD = 1.93) and in two samples of adolescents with ADHD (Study 2, Sample 1: n = 78; M[subscript age] = 12.12 years, SD = 0.91; Sample 2: n = 206; M[subscript age] = 15.35 years; SD = 0.85). A four-factor solution of the DERS-P was obtained in Study 1 and confirmed in Study 2, with factors demonstrating acceptable internal consistency. The community sample was generally rated as less dysregulated than the ADHD samples. Support was obtained for convergent, concurrent, and incremental validity evidence. These findings provide preliminary evidence for the DERS-P as a psychometrically sound parent-report measure of emotion dysregulation in 11- to 17-year-old adolescents. [This is the online version of an article published in "Assessment."]