We examine the value relevance of accounting information in relation to related-party transactions (RPTs), considering the type of RPTs and the controlling power of the owner. Based on an analysis of 11,651 firm-year observations, we find that the value relevance of RPTs is positive when the controlling power of the owner is weak. This suggests that investors perceive RPTs as efficient and beneficial for maximizing firm value when the ownership structure is not excessively concentrated. However, we find that RPTs are not value relevant when the controlling power of the owner is strong. This result implies that investors may consider RPTs as opportunistic behavior under high ownership concentration. These findings remain consistent even when categorizing RPTs based on their types (i.e., cash or non-cash). Moreover, we find that the value relevance of RPTs is more positive when firms belong to large business groups ("chaebols") and foreigner ownership is high. Overall, our study highlights the varying value relevance of RPTs depending on the controlling power of the owner, offering important implications for regulators, policymakers, and investors interested in RPTs.