Public administrations are increasingly relying on collaboration within inter-organizational networks to coordinate the development and provisioning of integrated digital public services. Collaboration strategies are necessary to focus collaborative efforts, align and prioritize goals, and leverage concrete results that advance integrated service delivery (ISD). However, literature on inter-organizational collaboration strategies for integrated service delivery is scarce. This research identifies collaboration strategies in three qualitative case studies in Belgium. The cases present 33 collaboration strategies categorized into engagement, motivation, and joint action strategies. Collaboration strategies should complement each other. Together these strategies can help to overcome the adoption paradox, one of the key issues to advance ISD, which refers to service providers not being inclined to adopt an ISD unless it has many users, but users might not be willing to use and ISD before many service providers have adopted it. Policy-makers can use the identified collaboration strategies to advance integrated service delivery.