Data about feminicide can be conceived as missing data – data that are neglected by authorities and underreported in official registries. Activists and civil society groups in Latin America and beyond are increasingly stepping into these gendered data gaps to undertake feminist counterdata science – an explicit challenge to the inadequate data practices of governments and judicial systems on the topic of gender-based killings. This chapter outlines opportunities for human-centred computing to design technologies that support and sustain counterdata collection efforts. It characterizes findings and design implications from twenty-one interviews with groups that monitor gender-based violence; enumerates ideas that arose from a participatory design process with activists; and describes two counterdata collection tools that build on this empirical work and attempt to operationalize feminist principles in their design. Throughout, the chapter calls for an explicitly intersectional feminist approach to co-designing tools to infrastructure counterdata science in the service of justice and liberation.