The worldwide increase in societal challenges is putting pressure on humanitarian organizations to develop sophisticated approaches to leverage social innovations in the humanitarian sector. Since humanitarian problems are complex problems, with the relevant knowledge being hidden, organizational search theory advocates the application of bottom-up and theory-guided search processes to identify the social innovations that solve these. Unfortunately, there has been no theoretical attention to understanding which approaches apply in this context. Further, established theory-guided bottom-up search processes, such as the lead user method, are unsuitable to the humanitarian sector, and we lack practice examples of adequate search processes. To start addressing this gap in theory and practice, procedural action research was done with the International Federation of Red Cross and Red Crescent Societies to develop a theory-guided bottom-up innovation search process for the real-life humanitarian problem of recurring floods in Indonesia. It revealed that an innovation search process for this context must differ significantly concerning its objectives and the steps to be taken from the lead user method, which was used as a starting point. Further, a comparison of the technical quality and the social impacts of the identified social innovations with social innovations identified through a non-theory-guided bottom-up search process (i.e., an innovation contest) suggests the superiority of this theory-guided search process. With this conclusion and the insights derived throughout the development of the search process, this study makes important contributions to theory development in the social and open innovation literatures and delivers important recommendations for social innovation practice in the humanitarian sector.