The main goal of this study was to explore the nature and structure of implicit theories of Spanish-speaking in-service teachers on learning to read. The study sample consisted of 591 in-service teachers from various Ibero-American countries (Spain, Mexico, Guatemala, Colombia, and Ecuador). The study analyzed attributional structure or teacher beliefs on learning to read based on principal component analysis. Findings revealed that many of the implicit theories on learning to read held by the teachers correspond to the historiography analysis and representational structure identified in previous studies. Based on main component analysis with varimax rotation, a factorial structure was found to be formed by the seven main factors related to the constructivist, nativist, maturationist, social, repetition, corrective, and psycho-linguistic learning theories. Also, significant differences were found in the learning theories of the teachers depending on geographical and cultural context. Spanish teachers were more likely to be associated with the psycho-linguistic and nativist theories. Mexican teachers were more closely associated with the constructivist theory, and both Guatemalan and Colombian teachers tended to be linked to the repetition-based theory. Finally, Ecuadorian teachers tended to rely on the nativist theory.