Cancer patients often experience poor quality of life (QoL) during chemotherapy (CT) treatments due to side effects including fatigue, insomnia, pain and nausea/vomiting. Mindfulness-based cancer recovery (MBCR) is an evidence-based intervention for treating such symptoms, but has not been investigated as an adjunctive treatment during CT. This study aims to determine the efficacy of an online group MBCR programme delivered during CT in 12 real-time interactive weekly sessions for managing fatigue (primary outcome). Secondary outcomes include sleep disturbance, pain, nausea/vomiting, mood, stress and QoL. Exploratory outcomes include cognitive function, white blood cell counts and return to work. The study is a two-armed randomised controlled waitlist trial with 2:1 allocation to treatment (online group MBCR during CT) or control (waitlist usual care; online MBCR following CT completion) with a target sample size of N = 178. Participants are breast or colorectal cancer patients undergoing common CT regimens in Calgary, Canada. Online assessments using validated self-reported instruments will take place at baseline, post-MBCR, post-CT and 12 months' post-baseline. If online MBCR delivered during CT significantly reduces fatigue in cancer patients' post-CT and also impacts secondary symptoms, this would provide evidence for including mindfulness training as an adjunctive symptom management therapy during CT.