This study investigates the effectiveness of advergames—digital games created specifically to advertise a brand—by examining the role of congruence between game rules and brand positioning. The authors explore how this alignment influences consumers' perceived legitimacy of the advergame, their attitudes toward the game and the brand, and ultimately their purchase intentions. Advergames have gained popularity as a marketing tool, but their effectiveness depends largely on the perceived relationship between the game's attributes (visual, narrative, and procedural) and the brand's characteristics. This study focuses on procedural congruence, which involves the alignment of the game's rules with the brand's positioning. Previous research has highlighted the benefits of congruence, particularly in terms of visual and thematic elements, but few studies have explored procedural congruence, leading to mixed results in the literature. The authors conducted a 2x2 experiment to test their hypotheses, manipulating the congruence between brand positioning (cooperative vs. non-cooperative) and game rules (cooperative vs. competitive). A fictitious brand and advergame were used to control for brand familiarity and biases. Participants were shown screenshots of the advergame with cooperative or competitive rules and then answered questions regarding perceived legitimacy, attitudes, and purchase intentions. The results confirmed that higher congruence between game rules and brand positioning led to greater perceived legitimacy of the advergame. This, in turn, positively influenced affective attitudes toward both the game and the brand, and subsequently increased purchase intentions. The study found that procedural congruence can legitimize advergames, making the advertising practice more acceptable and effective. The serial mediation model demonstrated that the effect of congruence on purchase intentions was entirely mediated by perceived legitimacy and affective attitudes. From an academic standpoint, this research extends the literature on advergames by showing that game rules, in addition to visual and narrative elements, play a critical role in advergame effectiveness. It also contributes to the understanding of advertising legitimacy, highlighting its importance in shaping consumer attitudes and behaviors. For practitioners, the study emphasizes that advergames should not be designed randomly. Advertisers should carefully align game elements, particularly the rules, with the brand's positioning to enhance legitimacy and positive consumer responses. The study’s limitations include the focus on cooperative versus competitive rules, the use of a fictitious brand, and the exclusion of other types of congruence such as visual or thematic alignment. Future research could explore other game genres and brand positioning strategies.