The objective of the study is to determine the efficacy of a 12-week mobile chat-based exercise counselling intervention (ChatEx) for increasing exercise behaviour (operationalized as self-reported aerobic MVPA) at 3-month (primary analysis, post-intervention) and 6-month (3-month post-intervention) among older cancer survivors. This will be a randomized controlled, assessor-blind trial conforming to the CONSORT guidelines. A total of 196 cancer survivors aged over 60 who have completed curative treatment and screened as having an ECOG performance status of 0 or 1 will be recruited and randomized into intervention and control groups. It is expected that upon intervention completion, the intervention group will exhibit increased and sustained exercise behaviors compared to the control group. Study instruments will be Physical Activity Scale for the Elderly, 6-minute walk test, grip strength, Edmonton Frail Scale, European Organization for Research and Treatment of Cancer Quality of Life Core Questionnaire, M-PAC constructs, and a background questionnaire. The intervention comprises of 12 weeks of messages aimed at promoting physical activity structured with the multi-process action control framework complemented with two face-to-face exercise instructional sessions at week 1 and week 6. The control group participants will receive a one-page written information sheet on the recommended exercise guidelines and exercise safety precautions in cancer survivors. Also, they will receive 3-4 messages via IM monthly on non-exercise-related self-management tips of cancer survivorship. They can receive the ChatEx intervention upon completion of the study. Intention-to-treat analysis will be performed.