The purpose of this study was to investigate the effect of individual and organisational factors on knowledge workers' perceptions of the effectiveness of informal learning. Because most studies on informal learning have been conducted at the individual level, studies empirically examining the influence of individual and organisational factors simultaneously remain necessary. The research question guiding this study was: What are organizational and individual factors that influence knowledge workers' perceptions of the effectiveness of informal learning? Tynjälä's (2013) 3-P (Presage, Process, and Product) model of workplace learning was adopted to identify factors affecting informal learning and its outcomes. In performing our multilevel analysis, we used the Human Capital Corporate Panel (HCCP) dataset covering a sample of 4628 knowledge workers across 396 companies in South Korea. At the individual level, task uncertainty, motivation, organisational commitment, job satisfaction, and perceived promotion opportunities were positively related to the workers' perceptions of the effectiveness of informal learning. At the organisational level, training and development (T&D) support and trust climate were significantly related to perceptions of the effectiveness of informal learning, while top management support and open communication were not.