Purpose: This systematic review aims to summarize eHealth studies with mindfulness- and relaxation-based interventions for medical conditions and to determine whether eHealth interventions have positive effects on health. Method: A comprehensive search of five databases was conducted for all available studies from 1990 to 2015. Studies were included if the intervention was mainly technology delivered and included a mindfulness- or relaxation-based intervention strategy and if patients with a medical condition were treated. Treatment effects were summarized for different outcomes. Results: A total of 2383 records were identified, of which 17 studies with 1855 patients were included in this systematic review. These studies were conducted in patients with irritable bowel syndrome, chronic fatigue syndrome, cancer, chronic pain, surgery, and hypertension. All but one study were delivered online through a web-based platform; one study delivered the intervention with iPods. The studies indicate that mindfulness- and relaxation-based eHealth interventions can have positive effects on patients' general health and psychological well-being. No effects were found for stress or mindfulness. Only five studies reported economic analyses of eHealth interventions without any clear conclusion. Conclusion: There is some evidence that mindfulness- and relaxation-based eHealth interventions for medical conditions can have positive effects on health outcomes. Therefore, such interventions might be a useful addition to standard medical care. No app studies were retrieved, even though a vast number of smartphone apps exist which aim at increasing users' health. Therefore, more studies investigating those health apps are needed.