IMPORTANCE: Family and other unpaid care partners may bridge accessibility challenges in interacting with the patient portal, but the extent and nature of this involvement is not well understood. OBJECTIVE: To inform an emerging research agenda directed at more purposeful inclusion of care partners within the context of digital health equity by (1) quantifying care partners' uptake and use of the patient portal in adolescent and adult patients, (2) identifying factors involving care partners' portal use across domains of the System Engineering Initiative for Patient Safety model, and (3) assessing evidence of perceived or actual outcomes of care partners' portal use. EVIDENCE REVIEW: Following Arksey and O'Malley's methodologic framework, a scoping review of manuscripts published February 1 and March 22, 2022, was conducted by hand and a systematic search of PubMed, PsycInfo, Embase, and Web of Science. The search yielded 278 articles; 125 were selected for full-text review and 41 were included. FINDINGS: Few adult patient portal accounts had 1 or more formally registered care partners (<3% in 7 of 7 articles), but care partners commonly used the portal (8 of 13 contributing articles reported >30% use). Care partners less often authored portal messages with their own identity credentials (<3% of portal messages in 3 of 3 articles) than with patient credentials (20%-60% of portal messages in 3 of 5 articles). Facilitators of care partner portal use included markers of patient vulnerability (13 articles), care partner characteristics (15 articles; being female, family, and competent in health system navigation), and task-based factors pertaining to ease of information access and care coordination. Environmental (26 articles) and process factors (19 articles, eg, organizational portal registration procedures, protection of privacy, and functionality) were identified as influential to care partner portal use, but findings were nuanced and precluded reporting on effects. Care partner portal use was identified as contributing to both patient and care partner insight into patient health (9 articles), activation (7 articles), continuity of care (8 articles), and convenience (6 articles). CONCLUSIONS AND RELEVANCE: In this scoping review, care partners were found to be infrequently registered for the patient portal and more often engaged in portal use with patient identity credentials. Formally registering care partners for the portal was identified as conferring potential benefits for patients, care partners, and care quality.