Background Globally, obesity is a growing crisis. Despite obesity being preventable, over a quarter of the UK adult population is currently considered clinically obese (typically body mass index ≥35 kg/m2). Access to treatment for people with severe obesity is limited by long wait times and local availability. Online and group-based interventions provide means of increasing the accessibility of obesity prevention and treatment services. However, there has been no prior review of the effectiveness of group-based interventions delivered online for people with severe obesity. Objective The purpose of this systematic review protocol is to provide an evaluation of the effectiveness and usability of different types of online, group-based interventions for people with severe obesity. Methods The Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P) and the Population, Intervention, Comparator, Outcome, and Study (PICOS) frameworks were used to structure this review. The review will systematically search 7 databases: MEDLINE, Embase, the Cumulative Index of Nursing and Allied Health Literature, APA PsycNet, Web of Science, CENTRAL, and the ProQuest Dissertations and Theses databases. Two authors (MM-I and LB) will independently screen the titles and abstracts of identified articles, select studies for inclusion based on the eligibility criteria, and extract data into a standardized form. Any disagreements will be discussed and resolved by a third reviewer (EM) if necessary. Risk of bias will be assessed using the Cochrane Collaboration Risk of Bias 2 tool and a descriptive analysis will be used to evaluate effectiveness and usability. Results The systematic review has not yet been started. It is expected to be completed and submitted for publication by December 2021. Conclusions This systematic review will summarize the effectiveness and usability of online, group-based interventions for people with obesity. It will identify the types of online delivery that have the strongest support to help inform the development of more useful and engaging interventions for people with severe obesity. Trial Registration National Institute for Health Research, PROSPERO CRD42021227101; https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021227101 International Registered Report Identifier (IRRID) PRR1-10.2196/26619