Humans have altered the majority of Earth’s terrestrial surface, yet some places still remain relatively undisturbed by modern society. These wilderness areas contain the most intact ecosystems on Earth. Here, we review the emerging evidence that wilderness areas are exceptionally important relative to more degraded ecosystems for biodiversity conservation (e.g. halving species extinction risk), Earth system functioning (e.g. supporting continental scale hydrological cycles), and for supporting the cultural integrity of many indigenous communities worldwide. We then describe the current state of wilderness conservation and mapping. Despite their immense value, wilderness areas are being rapidly lost, are under protected, and are almost completely overlooked in global environmental policy. Our window of opportunity to safeguard Earth’s last wilderness areas and their unique values is closing fast, but through a combination of smart policy changes and immediate large-scale conservation efforts, we can still secure them for future generations.