BACKGROUND: Mindfulness-based therapies (MBTs) addressed to patients with cancer have been widely studied in the last two decades, and their efficacy has been systematically reviewed and meta-analysed. Although findings from literature highlight benefits of MBTs on several patients' health outcomes, these should be appraised taking into consideration the characteristics of the selected studies. In this systematic review, we summarised the current evidence of the efficacy of MBTs in improving the quality of life of both patients with cancer and their relatives, with a focus on the methodological quality, type of MBT evaluated and population involved in existing randomised controlled trials (RCTs). METHODS: We searched English language articles published until February 2021. Couples of authors independently applied inclusion criteria and extracted findings. Thirty RCTs were included. RESULTS: Nearly half of the studies were performed in English-speaking countries outside of Europe, with females diagnosed with breast cancer. Most considered heterogeneous phases of illness; one study only was performed on relatives. In most cases, different measures were employed to evaluate the same outcome. The efficacy of MBTs has been demonstrated in 25 of the 30 included articles. The methodological quality of RCTs was acceptable. CONCLUSION: The heterogeneity of studies' characteristics makes findings on the efficacy of MBTs poorly informative with reference to different clinical and cancer-related psychological conditions. Studies on more homogeneous samples by cancer site and phase, as well as performed in different cultural contexts, could provide a basis for better evaluating and targeting MBTs' protocols for the specific needs of patients with cancer and their relatives.