Cities have classically been viewed as biologically impoverished, homogenized, and simplified systems that harbor low value for biodiversity. However, recent work has demonstrated that cities are critical ecological systems that provide important services to humanity and biodiversity. Cities can play vital roles in conserving biodiversity as well as habitats, providing system stability, and offering direct opportunities for people to engage with nature. In a rapidly changing world with biodiversity loss at an all-time high, cities have the potential to act as sanctuaries for biodiversity conservation. Here, we propose a “cities as sanctuaries” concept, which demonstrates the importance of cities for ecology and conservation, and provide the groundwork for advancing this novel field of study.