Background and Aim: Gamefulness is commonly cited as the primary goal of gamification, a family of approaches employed in education, business, healthcare, government, and elsewhere. However, gamefulness is defined imprecisely across the literature. To address this, we present a theory of gamefulness that splits gamefulness into more specific constructs and outlines their effects in a process model. Method: We integrate extant literature from psychology, human-computer interaction, and other fields to define gameful design, systems, and experiences. Most critically, we argue that gameful experience is the core focal construct of this theory and define it as an interactive state occurring when a person perceives non-trivial achievable goals created externally, is motivated to pursue them under an arbitrary set of behavioral rules, and evaluates that motivation as voluntary. Results: We present six resulting propositions: (1) gameful systems lead to gameful experiences, (2) gameful systems impact psychological characteristics, (3) effective gameful design leads to a gameful system, (4) gameful systems lead to behavioral change, (5) behavioral change causes the distal outcomes gamification designers target, and (6) individual differences moderate the effectiveness of gameful systems. Conclusion: Gameful experience theory provides researchers with a unified foundation to study gamification from any social scientific lens.