The current study examined how early smartphone ownership impacts parent-child informant discrepancy of youth internalizing problems during the transition to adolescence. We used four waves of longitudinal data (Years 1–4) from the Adolescent Brain Cognitive Development (ABCD; Baseline N = 11,878; White = 52.0%, Hispanic = 20.3%, Black = 15.0%, Asian = 2.1%, Other = 10.5%; Female = 47.8%). Across the full sample, significant parent-child informant discrepancy, such that parents underestimated child reports, appeared at Year 2 (Mage = 12.0) and increased across the remainder of the study (b = −0.21, SE = .042, p < .001, 95%CI [−.29, −.23]). Further, multi-group models indicated that significant parent-child informant discrepancy emerged in the years following initial smartphone acquisition, whereas youth who remained non smartphone owners did not demonstrate such a pattern. Moreover, this discrepancy grew with additional years of smartphone ownership. This study contributes to the ongoing discourse on adolescent smartphone use and mental health by documenting a novel, longitudinally observed risk to timely parental detection of mental health problems by early smartphone ownership.