Cancer survivors consider nutrition to be highly important and are motivated to seek information about lifestyle changes, including nutrition, to improve their long-term health. Despite this, suboptimal dietary intake is still reported. Understanding cancer-specific barriers and facilitators to healthy eating among this population could help develop targeted interventions for this group. This study explored the barriers and facilitators to following a healthy diet among cancer survivors using the Theoretical Domains Framework (TDF) and COM-B model. Semi-structured focus groups with 20 cancer survivors were conducted between February and August 2021, which were transcribed verbatim. Seven key TDF domains accounted for 87% of all coded responses. These were (a) environmental context and resources; (b) knowledge; (c) behavioural regulation; (d) social/professional role and identity; (e) belief about consequences; (f) social influences; and (g) skills. Key barriers were lack of knowledge, non-specific or irrelevant information, environmental aspects, and family. Facilitators were awareness of the importance of nutrition, the health benefits of a plant-based diet, confidence in cooking skills, organisation, balance, family, time, and viewing themselves as someone who eats healthily and access to fresh produce. Enablement was the most prominently linked intervention function. This work can inform future interventions in this area and ensure they are end-user-centred.