Almost all research on adolescents’ problematic social media use (PSMU) utilizes self-reports of symptoms. Recently, parent-report scales have been validated. Yet, in order to use parent-reports, it is imperative to understand the level of agreement between self- and parent-reported adolescents’ PSMU. This study examined agreement and discrepancies between adolescent-, mother-, and father-reported adolescents’ PSMU (assessed with the Social Media Disorder scale) on classification, the overall number of symptoms, and individual symptom level. Data from 234 Dutch adolescents aged 10–19 years and their parents (160 mothers and 91 fathers) from the Digital Family project were used. Configural invariance was established across all dyads and scalar invariance across mothers and fathers. Across adolescents and mothers/fathers, partial scalar invariance was found (one of the nine-item thresholds appeared noninvariant). Overall, we found poor agreement, but the level of agreement seems biased by the low prevalence of PSMU. Positive agreement was lower for symptoms that are harder to observe. Parental over- and underreporting compared to adolescent self-reports on the number of PSMU symptoms occurred to the same extent. Moreover, parental over- and/or underreporting were related to the number of adolescent- and parent-reported adolescents’ PSMU symptoms, adolescents’ gender, and maternal worrying about children’s social media/gaming addiction. Researchers should be aware that self- and parent-reports are not (always) interchangeable.