National parks and other protected areas (PAs) are the foundation of global efforts to conserve biological diversity. Conservation policy and practice assume that PAs are permanent fixtures on the landscape, but scattered evidence points to widespread—yet largely overlooked—PA downgrading, downsizing, and degazettement (PADDD). As a preliminary investigation of PADDD and its implications for conservation science and policy, we explore the published literature and contemporary media reports. We identify 89 historic instances of PADDD, in 27 countries, since 1900. Contemporary accounts reveal that PADDD has recently occurred or is currently under consideration in at least 12 countries worldwide. Proximate causes of PADDD vary widely, but center on access to and use of natural resources. Case studies from India and South America highlight the fact that PAs are socially defined and socially constructed governance regimes, responsive to social pressures—including conservation demands—at local to global scales. PADDD challenges longstanding assumptions underlying conservation policy and practice, including efforts to reduce deforestation and forest degradation (REDD), and underscores the need for resilient and robust conservation strategies. Because many fundamental questions regarding PADDD remain unanswered, further research is required to understand this conservation phenomenon and develop tailored policy responses.