Environmental change is rapidly accelerating, and many species will need to adapt to survive¹. Ensuring that protected areas cover populations across a broad range of environmental conditions could safeguard the processes that lead to such adaptations^1,2,3. However, international conservation policies have largely neglected these considerations when setting targets for the expansion of protected areas⁴. Here we show that—of 19,937 vertebrate species globally^5,6,7,8—the representation of environmental conditions across their habitats in protected areas (hereafter, niche representation) is inadequate for 4,836 (93.1%) amphibian, 8,653 (89.5%) bird and 4,608 (90.9%) terrestrial mammal species. Expanding existing protected areas to cover these gaps would encompass 33.8% of the total land surface—exceeding the current target of 17% that has been adopted by governments. Priority locations for expanding the system of protected areas to improve niche representation occur in global biodiversity hotspots⁹, including Colombia, Papua New Guinea, South Africa and southwest China, as well as across most of the major land masses of the Earth. Conversely, we also show that planning for the expansion of protected areas without explicitly considering environmental conditions would marginally reduce the land area required to 30.7%, but that this would lead to inadequate niche representation for 7,798 (39.1%) species. As the governments of the world prepare to renegotiate global conservation targets, policymakers have the opportunity to help to maintain the adaptive potential of species by considering niche representation within protected areas^1,2.