The world's biomes and their associated ecosystems are artificially fractured by geopolitical boundaries that define countries. Yet 'transboundary' landscapes often overlap with biodiversity hotspots, contain surprisingly important ecosystems, and provide critical habitats for threatened species. Notwithstanding, biodiversity in these landscapes is increasingly imperiled by infrastructure, including walls and fences along borders and cross-border roads that drive landscape fragmentation and the loss of ecological connectivity. Associated problems due to reduced governance (e.g., illegal wildlife trade) also undermine conservation efforts in these important regions. In this review, we distinguish positive and negative effects of transboundary frontiers on biodiversity conservation, discuss lessons from existing frameworks, and identify scenarios that can maximize opportunities for biodiversity conservation in transboundary frontiers.