Objective: The aim was to evaluate the effects of cocreated internet-based stepped care (iCAN-DO) on anxiety, depression, posttraumatic stress, and health-related quality of life (HRQoL) in individuals with cancer and self-reported anxiety and/or depression symptoms, compared with standard care. Methods: Clinically recruited individuals with breast, colorectal, or prostate cancer underwent online screening with the Hospital Anxiety and Depression Scale (HADS). Those with anxiety and/or depression symptoms (>7 on any of the HADS subscales) were randomized to iCAN-DO or standard care. iCAN-DO comprised psychoeducation and self-care strategies (step 1) and internet-based cognitive behavioral therapy (iCBT, step 2). Data were collected before randomization and at 1, 4, 7, and 10 months and analyzed with intention-to-treat regression analysis and randomization tests. Results: Online screening identified 245 (27%) of 909 individuals who reported anxiety and/or depression symptoms. They were randomized to iCAN-DO (n = 124) or standard care (n = 121). Of them 49% completed the 10-month assessment, and in the iCAN-DO group 85% accessed step 1 and 13% underwent iCBT. iCAN-DO decreased the levels of symptoms of depression (−0.54, 95% confidence interval: −1.08 to −0.01, P < .05) and the proportion of individuals with symptoms of Johansson Birgitta and Igelström Helena should be considered joint senior authors.