Increased popularity of smartphones has great potential to assist individuals in making healthy changes to their behaviour. Obesity and sedentary lifestyles are fast becoming a healthcare concern in modern societies. This study takes an important step in designing persuasive fitness applications (or apps) that can enhance the physical activity behaviour of individuals. We conceptualize that persuasive technology design principles embedded in social cognitive theory constructs will lead to the design of successful fitness apps. The conceptual model will be tested by analysing the design principles in existing mobile fitness apps. The theoretical and practical implications are discussed.