Biodiversity is declining faster than at any time in human history, with anthropogenic drivers causing significant alteration of 75% of the world’s land surface. The resultant habitat loss is driving more species than ever towards extinction, with range-restricted, endemic species most at risk. Nevertheless, it is unrealistic to believe that development will suddenly cease; thus, achieving conservation goals whilst simultaneously promoting sustainable development requires integrated landscape planning, spatial prioritization, and the enforcement of appropriate legislation. Here, we introduce an open-access, online tool that maps the locations of these most imperiled of species within South Africa. The Threatened Species No-Go Mapping Tool’s primary objective is to inform the placement of proposed potentially detrimental developments in order to encourage early-stage Environmental Impact Assessment rejection or avoidance. Furthermore, from an applied conservation viewpoint, the tool allows one to assess the extent to which these No-Go Areas fall within the country’s Protected Area network. We show how this information can be used to focus conservation efforts and guide protected area expansion plans, ultimately helping in safeguarding the most vulnerable species. Finally, we discuss how the tool can be adapted and adopted in other regions/countries with high levels of range-restricted, endemic species. Solution: By mapping the distribution of the most range-restricted, threatened species, the Threatened Species No-Go Mapping Tool informs the placement of proposed potentially detrimental developments in order to encourage early-stage Environmental Impact Assessment rejection or avoidance. As a result, unnecessary cost to the developer and the responding interested and affected parties can be avoided, ultimately reducing the irreversible loss of critical habitat.