Startups play a crucial role in advancing the Sustainable Development Goals (SDGs) through their innovative solutions that increasingly focus on sustainability. However, they face significant challenges in effectively assessing their contribution to the SDGs. In our study, by adopting an action-research methodology, we develop and introduce Prosper, i.e., a tailored social impact assessment (SIA) framework for startups. First, we conducted a comprehensive review of existing methods and standards for assessing social impact to establish an initial foundation of Prosper. Second, we refined Prosper based on an empirical validation on five startups. By embracing action research, we aim to empower startups with a robust and user-friendly tool, which facilitates SIA and representation. We also contribute to the literature aimed at overcoming the existing sustainability barriers for startups and to respond to the call for assisting them in reporting about SDGs. We also discuss practical implications and future research avenues.