Physical activity after a cancer diagnosis has multiple benefits, but many people living with and beyond cancer are not meeting physical activity guidelines. Apps are a promising way to help people increase their physical activity. However, there have been few trials in this area. This protocol paper describes a pilot trial of an app that promotes brisk walking for patients with breast, prostate or colorectal cancer. This pilot study will inform the design of a full randomised controlled trial by seeing whether the trial design is practical and acceptable to the patients who volunteer to take part.