The spatial configuration of cities can affect how urban environments alter local energy balances. Previous studies have reached the paradoxical conclusions that both sprawling and high-density urban development can amplify urban heat island intensities, which has prevented consensus on how best to mitigate the urban heat island effect via urban planning. To investigate this apparent dichotomy, we estimated the urban heat island intensities of the 50 most populous cities in the United States using gridded minimum temperature datasets and quantified each city's urban morphology with spatial metrics. The results indicated that the spatial contiguity of urban development, regardless of its density or degree of sprawl, was a critical factor that influenced the magnitude of the urban heat island effect. A ten percentage point increase in urban spatial contiguity was predicted to enhance the minimum temperature annual average urban heat island intensity by between 0.3 and 0.4°C. Therefore, city contiguity should be considered when devising strategies for urban heat island mitigation, with more discontiguous development likely to ameliorate the urban heat island effect. Unraveling how urban morphology influences urban heat island intensity is paramount given the human health consequences associated with the continued growth of urban populations in the future. •Urban heat island (UHI) intensities were estimated for the fifty most populous cities in the USA using PRISM climate data.•The urban morphologies of the cities were quantified using spatial metrics and the NLCD 2006 land use/land cover dataset.•The statistical analyses suggested that highly contiguous dense and sprawling urban development both enhance the UHI effect.•City contiguity should be considered when devising strategies for UHI intensity mitigation.•More discontiguous city configurations, especially if achieved by introducing urban green spaces, will likely reduce UHIs.