Privately protected areas increase global protected area coverage and connectivity.

Privately protected areas (PPAs) are increasing in number and extent. Yet, we know little about their contribution to conservation and how this compares to other forms of protected area (PA). We address this gap by assessing the contribution of 17,561 PPAs to the coverage, complementarity and connectivity of existing PA networks in 15 countries across 5 continents. We find that PPAs (1) are three times more likely to be in biomes with <10% of their area protected than are other PA governance types and twice as likely to be in areas with the greatest human disturbance; (2) that they protect a further 1.2% of key biodiversity areas; (3) that they account for 3.4% of land under protection; and (4) that they increase PA network connectivity by 7.05%. Our results demonstrate the unique and significant contributions that PPAs can make to the conservation estate and that PPAs deserve more attention, recognition and resources for better design and implementation.