The aim of this thesis in Interaction Design is to understand how online federation configures alternative social media (ASM). I use the ecosystem of the Fediverse, a network of interoperable alternative social media platforms, as a case study. Drawing on theories of configuration—that concepts, technologies, users, and their use practices are coconstitutive—the research explores how online federation has been used, imagined, and re-imagined in the design of ASM. Through a series of prototypes informed by Participatory Design, I engage in (re)configuring off-the-shelf ASM together with stakeholders. Drawing from that practice-based inquiry, this study provides insights into the ways online federation as commonly configured affects platform design and counteracts stakeholder intentions. Along with research insights, the prototypes yielded tangible technological contributions in the form of advanced working prototypes usable by the partners,improvements to code bases of existing ASM projects, and feedback for project developers. In paying attention to online federation as both a set of ideas and technical affordances,along with their impact on data circulation on federated platforms, the thesis develops an understanding of federation’s implications for platform design and content moderation. In doing so, the thesis contributes to existing gaps in scholarship on how federation can be used to create new avenues for co-ownership and co-design of the online infrastructures we rely on.