This study investigates an alliance formation between competing manufacturers and a monopolistic platform retailer, where consumer data collected by the retailer are available between all the alliance members to utilize for the development of their products. The manufacturers alternatively choose to adopt the direct-to-consumer model, where they collect and utilize own consumer data or sell products without data utilization. They are differentiated from each other in varying degree of substitutability. Under the alliance, the effectiveness of utilizing their rival’s consumer data is proportional to the degree of substitutability. By using a game-theoretic approach, we analyze a strategic interaction between two or three manufacturers and the retailer to derive the condition under which the alliance is successfully formed in equilibrium and to discuss the retailer’s profitability of the alliance formation.We show that participation in the alliance has a direct effect of sharing their rivals’ consumer data and an indirect effect of mitigating price competition between the manufacturers due to the “free-riding” effect of data sharing. Despite these merits, the alliance may not be formed in equilibrium. When the two manufacturers are differentiated to some extent or the three manufacturers are symmetrically differentiated, the alliance is successfully formed. Otherwise, it is never formed in equilibrium. We show that the retailer benefits the most from the alliance with two intermediately differentiated manufacturers and that an increase in the number of manufacturers participating in the alliance does not necessarily improve the retailer’s profitability.