OBJECTIVE: Internet-delivered interventions may alleviate distress in cancer survivors with limited access to psychological face-to-face treatment. In collaboration with a group of cancer survivors, we developed and tested the efficacy of a therapist-assisted internet-delivered mindfulness-based cognitive therapy (iMBCT) program for anxiety and depression in cancer survivors. METHODS: A total of 1282 cancer survivors were screened for anxiety and depression during their routine oncology follow-up; eligible breast (n = 137) and prostate cancer (n = 13) survivors were randomized to iMBCT or care-as-usual (CAU) wait-list. Primary outcomes of anxiety and depression were assessed at baseline, 5 weeks, 10 weeks (post intervention), and 6 months. RESULTS: Significant effects were found for both anxiety (Cohen's d = 0.45; P = .017) and depressive symptoms (d = 0.42; P = .024) post intervention. The effects were maintained at follow-up for anxiety (d = 0.40; P = .029), but not for depressive symptoms (d = 0.28; P = .131). CONCLUSIONS: Our preliminary findings suggest iMBCT to be a helpful intervention for cancer survivors suffering from symptoms of anxiety. Further studies on the efficacy for symptoms of depression are needed.