Interventions targeting diet and physical activity have demonstrated to be effective for improving glycaemic control in adults with type 2 diabetes. However, initiating and sustaining these changes remains a challenge. Ingestion of whey protein has shown to be effective for improving glycaemic control by increasing insulin and incretin secretion, and influencing appetite regulation, however little is known about what influences uptake and adherence. We conducted a qualitative interview study to explore behavioural determinants of uptake and adherence to a commercially made whey protein supplementation. 16/18 adults with type 2 diabetes who participated in an RCT took part in a semi-structured interview. Seven themes were generated from the data following thematic analyses. The most frequently reported determinant of uptake was the expectation that the supplement would improve health status (e.g., type 2 diabetes management), as a consequence of appetite suppression and weight loss. Determinants of adherence included palatability; the belief that the supplement was an appetite suppressant; and receiving positive reinforcement on the effects of the supplement. Frequency of consumption led to reduced adherence with some participants. Findings support that the whey-protein supplement is a viable management option for adults with type 2 diabetes, however uptake will be driven by conveying information on the positive effects of the supplement on appetite suppression, and glycaemic control. Adherence will be determined by palatability, behavioural prompting, and positive reinforcement.