Business value of IT is an enduring research question. The elusive link between IT and financial firm performance calls for further research into intermediate organizational variables through which IT may influence firm performance. This study proposes that knowledge management (KM) is a critical organizational capability through which IT influences firm performance. In the context of multibusiness firms, the study examines how the IT resources of a firm should be organized and managed to enhance the firm’s KM capability, and whether and how KM capability influences firm performance. The study develops two hypothesizes: (1) IT relatedness of the firm’s business units enhances cross-unit KM capability; (2) KM capability, in turn, leads to superior firm performance. Data from 250 Fortune1000 firms provide empirical support for these hypotheses. IT relatedness of business units enhances the cross-unit KM capability of the firm. The KM capability creates and exploits cross-unit synergies from the product, customer, and managerial knowledge resources of the firm. These synergies increase the financial performance of the firm. IT relatedness also has significant indirect effects on firm performance through the mediation of KM capability.