Cities are investing billions of dollars in climate change adaptation to combat the effects of sea‐level rise, temperature extremes, increasingly intense storm events, flooding and water scarcity. Natural ecosystems have enormous potential to contribute to city resilience, and so, actions that rely on this approach could sustain considerable co‐benefits for biodiversity. In this paper we identify the prevalence of key themes of human adaptation response that could have biodiversity conservation outcomes in cities. We then quantify the area of impact for actions that identify specific targets for greening or green infrastructure that could involve natural ecosystems, providing an indicator of potential co‐benefits to biodiversity. We then extrapolate to explore the total area of land that could benefit from catchment management approaches, the area of waterways that could benefit from nature‐based improvement of these spaces, and finally the number of threatened species that could benefit across these cities. From 80 city climate adaptation plans analysed, we found that urban greening plays a key role in most adaptation strategies, and represents an enormous opportunity for biodiversity conservation, given the diversity of animal and plant species in urban environments. We show that the ranges of at least 270 threatened species overlap with the area covered by just 58 city adaptation plans, including watershed catchments totalling over 28 million km2. However, an analysis of 80 city adaptation plans (of a total 151 found globally) shows that this opportunity is being missed. Just 18% of the plans assessed contained specific intentions to promote biodiversity. We highlight this missed opportunity, as climate adaptation actions undertaken by cities represent an enormous incipient opportunity for nature conservation. Finally, we encourage planners and city governments to incorporate biological conservation into climate adaption plans, for the mutual benefit of urban societies and their biodiversity.