Over the past decade, regions worldwide have developed initiatives to tackle climate change adaptation at the metropolitan scale. Faced with barriers to adaptation planning and implementation at the local level, growing numbers of practitioners have turned to the metropolitan region as a new scalar platform for climate adaptation planning. This study examines regional adaptation planning in Los Angeles, Miami, and Boston, three metropolitan areas that have significant exposure to the impacts of climate change and typify the high levels of administrative fragmentation found in the United States. I ask, what regional adaptation strategies have they deployed? What local adaptation challenges do they try to overcome? Given their progress, what are the implications for this scale of adaptation planning? Drawing on case studies of these efforts, I End that regional collaborations promisingly get more cities to start planning for the impacts of climate change, even in states where governments oppose climate action, by increasing access to information, providing opportunities for networking and technical assistance, helping secure additional funding, and strengthening coordination among vertical levels of government. However, local-centric regional adaptation efforts have had less success in addressing horizontal coordination challenges across municipalities, particularly as they relate to land use planning, fiscal constraints, spillovers effects, and social equity. These findings suggest that scaling up adaptation to the metropolitan region is no panacea for overcoming structural limits to local adaptation in places with weak regional governance institutions. More critical and reflexive regional adaptation planning can help pave the way to difficult conversations around regional institution building so that governance at this scale can achieve its promise of producing more effective and equitable adaptation.