Background Patient portals are digital health tools adopted by health care organizations. The portals are generally connected to the electronic health record of the health care organization and offer patients functionalities such as access to the medical record, ability to order repeat prescriptions, make appointments, or message the health care provider. Patient portals may be beneficial for both patients and the health care system. Patient portals can widely differ from one context to another due to the differences in the portal functionalities and capabilities and it is anticipated that outcomes associated with the functionalities also differ. Current systematic reviews report outcomes associated with patient portal uptake but do not explicitly specify the patient portal functionalities. Objective The aim of this systematic review is to synthesize the evidence on health and health care quality outcomes associated with patient portal use among adult (18 years or older) patients. The review research questions are as follows: What kind of health outcomes do tethered patient portals and patient portal functionalities contribute to in adult patients (18 years or older)? and What kind of health care quality outcomes, including health care utilization outcomes, do tethered patient portals and patient portal functionalities contribute to in adult patients (18 years or older)? Methods The systematic review will be conducted by searching the MEDLINE, EMBASE, and Scopus databases for relevant literature. The review inclusion criteria will be studies about adult patients (18 years or older), studies only about tethered patient portals, and studies with or without a comparator. We will report patient portal–associated health and health care quality outcomes based on the patient portal functionalities. All quantitative primary study types will be included. Risk of bias of included studies will be assessed using the Cochrane Collaboration’s tool for assessing risk of bias in randomized trials and the National Heart, Lung, and Blood Institute’s quality assessment tools. Data will be synthesized using narrative synthesis and will be reported according to the patient portal functionalities, country, disease, and health care system model. Results Searches will be conducted in September 2019, and the review is anticipated to be completed by the end of June 2020. Conclusions This systematic review will provide an overview of health and health care quality outcomes associated with patient portal use among adult patients, providing detailed information about the functionalities of the portals and their associations with the outcomes. The review could potentially help patient portal evaluation studies by providing insights into outcomes associated with the different functionalities of patient portals. Trial Registration International Prospective Register of Systematic Reviews (PROSPERO) CRD42019141131; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=141131 International Registered Report Identifier (IRRID) PRR1-10.2196/14975