•A multilevel process theory of gameful experience is developed and presented.•The term gamefulness is replaced with three more precisely defined constructs.•Gameful design, gameful systems, and gameful experiences are carefully defined.•This theory links these constructs causally whereas previously they were confounded.•This theory thus serves as a unifying foundation for future work on gamification. 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. 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. We present six resulting propositions: (1) gameful systems lead to gameful experiences, (2) gameful systems impact psychological characteristics, (3) effective gameful design leads to gameful systems, (4) effective gameful systems lead to behavioral change, (5) appropriate behavioral change causes the distal outcomes gamification designers target, and (6) individual differences moderate the effectiveness of gameful systems. Gameful experience theory provides researchers with a unified foundation to study gamification from any social scientific lens.