BACKGROUND: Cancer has emerged as a considerable global health concern, contributing substantially to both morbidity and mortality. Recognizing the urgent need to enhance the overall well-being and quality of life (QOL) of cancer patients, a growing number of researchers have started using online mindfulness-based interventions (MBIs) in oncology. However, the effectiveness and optimal implementation methods of these interventions remain unknown. OBJECTIVE: This study evaluates the effectiveness of online MBIs, encompassing both app- and website-based MBIs, for patients with cancer and provides insights into the potential implementation and sustainability of these interventions in real-world settings. METHODS: Searches were conducted across 8 electronic databases, including the Cochrane Library, Web of Science, PubMed, Embase, SinoMed, CINAHL Complete, Scopus, and PsycINFO, until December 30, 2022. Randomized controlled trials involving cancer patients aged ≥18 years and using app- and website-based MBIs compared to standard care were included. Nonrandomized studies, interventions targeting health professionals or caregivers, and studies lacking sufficient data were excluded. Two independent authors screened articles, extracted data using standardized forms, and assessed the risk of bias in the studies using the Cochrane Bias Risk Assessment Tool. Meta-analyses were performed using Review Manager (version 5.4; The Cochrane Collaboration) and the meta package in R (R Foundation for Statistical Computing). Standardized mean differences (SMDs) were used to determine the effects of interventions. The Reach, Effectiveness, Adoption, Implementation, and Maintenance framework was used to assess the potential implementation and sustainability of these interventions in real-world settings. RESULTS: Among 4349 articles screened, 15 (0.34%) were included. The total population comprised 1613 participants, of which 870 (53.9%) were in the experimental conditions and 743 (46.1%) were in the control conditions. The results of the meta-analysis showed that compared with the control group, the QOL (SMD 0.37, 95% CI 0.18-0.57; P<.001), sleep (SMD -0.36, 95% CI -0.71 to -0.01; P=.04), anxiety (SMD -0.48, 95% CI -0.75 to -0.20; P<.001), depression (SMD -0.36, 95% CI -0.61 to -0.11; P=.005), distress (SMD -0.50, 95% CI -0.75 to -0.26; P<.001), and perceived stress (SMD -0.89, 95% CI -1.33 to -0.45; P=.003) of the app- and website-based MBIs group in patients with cancer was significantly alleviated after the intervention. However, no significant differences were found in the fear of cancer recurrence (SMD -0.30, 95% CI -1.04 to 0.44; P=.39) and posttraumatic growth (SMD 0.08, 95% CI -0.26 to 0.42; P=.66). Most interventions were multicomponent, website-based health self-management programs, widely used by international and multilingual patients with cancer. CONCLUSIONS: App- and website-based MBIs show promise for improving mental health and QOL outcomes in patients with cancer, and further research is needed to optimize and customize these interventions for individual physical and mental symptoms. TRIAL REGISTRATION: PROSPERO CRD42022382219; https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=382219.