BACKGROUND: Depression poses a major threat to public health with an increasing prevalence in the United States. Mindfulness-based interventions, such as mindfulness-based cognitive therapy (MBCT), are effective methods for managing depression symptoms and may help fortify existing efforts to address the current disease burden. The in-person group format of MBCT, however, incurs barriers to care such as expenses, childcare needs, and transportation issues. Alternate delivery modalities such as MBCT delivered via the web can be investigated for their capacity to overcome these barriers and still reduce symptoms of depression with adequate feasibility and efficacy. OBJECTIVE: This study protocol aims to examine the feasibility and efficacy of MBCT delivered via the web for the treatment of depression. METHODS: To attain study aims, 2 phases will be implemented using a waitlist control design. A total of 128 eligible participants will be randomized into either an 8-week MBCT intervention group plus treatment as usual (MBCT + TAU; group 1) or an 8-week waitlist control group (group 2). In phase I (8 weeks), group 1 will complete the intervention and group 2 will proceed with TAU. In phase II (8 weeks), group 2 will complete the intervention and group 1 will continue with TAU until reaching an 8-week follow-up. TAU may consist of receiving psychotherapy, pharmacotherapy, or combined treatment. Data collection will be completed at baseline, 8 weeks (postintervention for group 1 and preintervention for group 2), and 16 weeks (follow-up for group 1, postintervention for group 2). The primary outcomes will include (1) current, residual, or chronic depression symptoms and (2) psychiatric distress. Secondary outcomes will include perceived stress and facets of mindfulness. The feasibility will be measured by assessing protocol adherence, retention, attendance, and engagement. Finally, the extent of mindfulness self-practice and executive functioning skills will be assessed as mediators of intervention outcomes. RESULTS: This study began screening and recruitment in December 2022. Data collection from the first cohort occurred in January 2023. By November 2023, a total of 30 participants were enrolled out of 224 who received screening. Data analysis began in February 2024, with an approximate publication of results by August 2024. Institutional review board approval took place on September 11, 2019. CONCLUSIONS: This trial will contribute to examining mindfulness-based interventions, delivered via the web, for improving current, residual, or chronic depression symptoms. It will (1) address the feasibility of MBCT delivered via the web; (2) contribute evidence regarding MBCT's efficacy in reducing depression symptoms and psychiatric distress; and (3) assess the impact of MBCT on several important secondary outcomes. Findings from this study will develop the understanding of the causal pathways between MBCT delivered via the web and depression symptoms further, elucidating the potential for future larger-scale designs. TRIAL REGISTRATION: ClinicalTrials.gov NCT05347719; https://www.clinicaltrials.gov/ct2/show/NCT05347719. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): DERR1-10.2196/53966.