Organisations are increasingly recognising customer knowledge as a valuable asset that they can utilise to strengthen their competitive strategies. Many scholars have observed that firms can improve business performance by exploiting this knowledge. Hence, the primary objective of this study is to examine the influence of customer knowledge on business performance and the moderating role of firm size in the context of a developing country, Vietnam. A total of 331 employees and managers from various types and sizes of organisations participated in the survey. The research employed Cronbach's alpha, exploratory factor analysis (EFA), confirmatory factor analysis (CFA), structural equation model (SEM), and multi-group analysis methods to achieve the research objectives. The results indicate that customer knowledge dimensions have a significant impact on business performance, and firm size plays a moderating role in the effect. Based on the research results, the managerial implications, further research suggestions, and research limitations will be discussed.