Information systems outsourcing is one of the important issues that has emerged in the information systems field since the 1990s. In particular, there has been considerable recent interest in how to form a successful partnership between users and providers in information systems outsourcing. Accordingly, to identify the success factors of outsourcing partnerships, this study, based on the information systems partnership model proposed by Henderson (1990), (1) presents a causal model with mutual benefits, commitment, and trust as mediating variables, (2) tests this model using data from 225 domestic companies engaged in information systems outsourcing, and (3) compares the causal model with a competing model without mediating variables. The results demonstrated that the causal model proposed in this study is superior to the competing model in explaining the relationship between partnership and outsourcing success.