Background Patient portal use could help improve the care and health outcomes of patients with diabetes owing to functionalities, such as appointment booking, electronic messaging (e-messaging), and repeat prescription ordering, which enable patient-centered care and improve patient self-management of the disease. Objective This review aimed to summarize the evidence regarding patient portal use (portals that are connected to electronic health care records) or patient portal functionality use (eg, appointment booking and e-messaging) and their reported associations with health and health care quality outcomes among adult patients with diabetes. Methods We searched the MEDLINE, Embase, and Scopus databases and reported the review methodology using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Three independent reviewers screened titles and abstracts, and two reviewers assessed the full texts of relevant studies and performed data extraction and quality assessments of the included studies. We used the Cochrane Collaboration Risk of Bias Tool and the National Heart, Lung and Blood Institute (NHLBI) Study Quality Assessment Tool to assess the risk of bias of the included studies. Data were summarized through narrative synthesis. Results Twelve studies were included in this review. Five studies reported overall patient portal use and its association with diabetes health and health care quality outcomes. Six studies reported e-messaging or email use–associated outcomes, and two studies reported prescription refill–associated outcomes. The reported health outcomes included the associations of patient portal use with blood pressure, low-density lipoprotein cholesterol, and BMI. Few studies reported health care utilization outcomes such as office visits, emergency department visits, and hospitalizations. A limited number of studies reported overall quality of care for patients with diabetes who used patient portals. Conclusions The included studies mostly reported improved glycemic control outcomes for patients with diabetes who used patient portals. However, limitations of studying the effects of patient portals exist, which do not guarantee whether the outcomes reported are completely the result of patient portal use or if confounding factors exist. Randomized controlled trials and mixed-methods studies could help understand the mechanisms involved in health outcome improvements and patient portal use among patients with diabetes. Trial Registration International Prospective Register of Systematic Reviews (PROSPERO) CRD42019141131; https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42019141131. International Registered Report Identifier (IRRID) RR2-10.2196/14975