Pain experiences of youth with brain-based developmental disabilities are often overlooked and/or misinterpreted, increasing the risk for poor or inadequate pain assessment and management. Ample measures exist to assess acute and chronic pain, yet their utility and frequency of use in youth with brain-based developmental disabilities is unclear and available measures do not have strong measurement properties for this diverse group. This systematic review identified the scope of self-reported and observer-reported pain assessment in studies of youth (aged 3-24 years) with brain-based developmental disabilities (phase 1) and summarized other measures of pain-related functioning for acute and chronic pain (ie, physical, emotional, social, sleep, and quality of life, within the subset of quantitative studies focused primarily on pain, phase 2). A comprehensive search for English-language studies was conducted in August 2022 in Web of Science, CINAHL, MEDLINE, Cochrane CENTRAL, EMBASE, and APA PsychINFO (PROSPERO registration: CRD42021237444). A total of 17,029 unique records were screened. Of the 707 articles included in phase 1, most assessed chronic pain (n = 314; 62.0%) and primarily used observer-report (n = 155; 31%) over self-report (n = 67; 13%). Of the 137 articles included in phase 2, other outcomes assessed alongside pain intensity included motor ability (16.8%), adaptive functioning (11%), quality of life (8%), pain interference (6.6%), mental health (5.8%), and communication ability (2.9%). Cerebral palsy was the most common population in both phase 1 (n = 343; 48.5%) and phase 2 (n = 83; 59.7%). This review provides a foundational understanding of pain assessment in brain-based developmental disabilities and highlights continued inequities in holistic pain assessment for this population.