BACKGROUND: The Internet can be leveraged to provide disease management support, including medication adherence promotion that, when tailored, can effectively improve adherence to medications. The growing adoption of patient portals represents an opportunity to support medication management and adherence more broadly, but virtually no data exist about the real and potential impact of existing portals on these outcomes. OBJECTIVE: We sought to (1) understand who uses an existing patient portal and reasons for use and nonuse, (2) understand how portal users are using a portal to manage their medications, and (3) explore participants' ideas for improving portal functionality for medication management and adherence support. METHODS: A total of 75 adults with type 2 diabetes participated in a mixed-methods study involving focus groups, a survey, and a medical chart review. We used quantitative data to identify differences between portal users and nonusers, and to test the relationship between the frequency of portal use and glycemic control among users. We used qualitative methods to understand how and why participants use a portal and their ideas for improving its medication management functionality. RESULTS: Of the enrolled participants, 81% (61/75) attended a focus group and/or completed a survey; portal users were more likely than nonusers to participate in that capacity (Fisher exact test; P=.01). Users were also more likely than nonusers to be Caucasian/white (Fisher exact test; P<.001), have higher incomes (Fisher exact test; P=.005), and be privately insured (Fisher exact test; P<.001). Users also tended to have more education than nonusers (Mann-Whitney U; P=.05), although this relationship was not significant at P<.05. Among users, more frequent use of a portal was associated with better A1C (Spearman rho =-0.30; P=.02). Reasons for nonuse included not knowing about the portal (n=3), not having access to a computer (n=3), or having a family member serve as an online delegate (n=1). Users reported using the portal to request prescription refills/reauthorizations and to view their medication list, and they were enthusiastic about the idea of added refill reminder functionality. They were also interested in added functionality that could streamline the refill/reauthorization process, alert providers to fill/refill nonadherence, and provide information about medication side effects and interactions. CONCLUSIONS: Although there are disparities in patient portal use, patients use portals to manage their medications, are enthusiastic about further leveraging portals to support medication management and adherence, and those who use a portal more frequently have better glycemic control. However, more features and functionality within a portal platform is needed to maximize medication management and adherence promotion.