Background: eHealth mindfulness-based programs (eMBPs) are on the rise in complex oncology and palliative care. However, we are still at the beginning of answering the questions of how effective eMBPs are and for whom, and what kinds of delivery modes are the most efficient. Objective: This systematic review aims to examine the feasibility and efficacy of eMBPs in improving the mental health and well-being of patients with cancer, to describe intervention characteristics and delivery modes of these programs, and to summarize the results of the included studies in terms of moderators, mediators, and predictors of efficacy, adherence, and attrition. Methods: In total, 4 databases (PubMed, PsycINFO, Scopus, and Web of Knowledge) were searched using relevant search terms (eg, mindfulness, program, eHealth, neoplasm) and their variations. No restrictions were imposed on language or publication type. The results of the efficacy of eMBPs were synthesized through the summarizing effect estimates method. Results: A total of 29 published papers describing 24 original studies were included in this review. In general, the results indicate that eMBPs have the potential to reduce the levels of stress, anxiety, depression, fatigue, sleep problems, and pain, and improve the levels of mindfulness, posttraumatic growth, and some parameters of general health. The largest median of Cohen d effect sizes were observed in reducing anxiety and depression (within-subject: median -0.38, IQR -0.62 to -0.27; between-group: median -0.42, IQR -0.58 to -0.22) and facilitating posttraumatic growth (within-subject: median 0.42, IQR 0.35 to 0.48; between-group: median 0.32, IQR 0.22 to 0.39). The efficacy of eMBP may be comparable with that of parallel, face-to-face MBPs in some cases. All studies that evaluated the feasibility of eMBPs reported that they are feasible for patients with cancer. Potential moderators, mediators, and predictors of the efficacy, attrition, and adherence of eMBPs are discussed. Conclusions: Although the effects of the reviewed studies were highly heterogeneous, the review provides evidence that eMBPs are an appropriate way for mindfulness practice to be delivered to patients with cancer. Thus far, existing eMBPs have mostly attempted to convert proven face-to-face mindfulness programs to the eHealth mode. They have not yet fully exploited the potential of eHealth technology.