Purpose: The purpose of this study is to investigate the link between informal learning and employees' in-role performance and whether the mechanism through informal learning mediates the relationships between self-efficacy, job characteristics, trust and in-role performance. Design/methodology/approach: Based on data (n = 294) obtained from the firms with the Work--Learning Dual System in South Korea, a structural equation modeling (SEM) analysis was conducted. Findings: The findings indicated that trust and job characteristics affected informal learning and informal learning mediates the relationships of trust and job characteristics with job performance. Originality/value: The significant contributions of this study to the extant literature on informal learning are as follows: first, the present study investigates a mechanism and a mediating role of informal learning using SEM, while most previous studies in literature have employed qualitative research in informal learning. Second, this study explores the mediating role of informal learning between personal/job-related determinants of informal learning and in-role performance, which has not yet been examined in existing literature. Finally, this study provides practical implications regarding how organizations can facilitate more informal learning among employees to enhance their performance.