Using institutional theory, this study offers an integrated framework that describes the diffusion of business-to-business e-commerce within a country. The model specifies the role of national institutional frameworks, international institutional pressures, and market complexity in business-to-business e-commerce diffusion. We test this model using archival, cross-sectional data from 146 countries for the period from 2013 to 2016. The study also compares the roles of these factors in business-to-business e-commerce diffusion across developed and developing countries. The results suggest that national institutional frameworks, international institutional pressures, and market complexity contribute positively to business-to-business e-commerce diffusion and that the influence of these variables varies according to the degree of a country’s development. Theoretical, research, and managerial implications of the study are also discussed.