In today's rapidly evolving educational landscape, informal digital learning has become an integral part of adult language acquisition. This study investigates the mediating role of psychological capital perceptions in the relationship between academic self-efficacy and digital informal learning competence among 220 adult English as a Foreign Language learners recruited through convenience sampling at a foundation university in Turkey. Data were collected using validated measurement scales—including Digital Informal Learning, Psychological Capital, and Academic Self-Efficacy—to examine how self-efficacy beliefs influence learners’ engagement with digital tools for informal learning, and how psychological capital (comprising self-efficacy, optimism, hope, and resilience) contributes to this process. Quantitative data were analyzed using mediation analysis to examine the indirect effects of academic self-efficacy on digital informal learning competence through psychological capital. The findings suggest that psychological capital serves as a significant mediator, enhancing learners' confidence and adaptability in digital learning environments. These insights highlight the importance of fostering psychological capital to improve self-directed learning and academic success. The study contributes to the growing body of research on digital learning strategies and psychological factors in language education, offering practical implications for educators and policymakers aiming to optimize informal digital learning experiences.