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Synopsis Urbanization promotes the formation of heat islands. For ectothermic animals in cities, the urban heat island effect can increase developmental rate and result in smaller adult body size (i.e., the temperature-size rule). A smaller adult body size could be consequential for invasive urban ectotherms due to potential effects of body size on thermal tolerance, dispersal distance, and fecundity. Here, we explored the effect of urbanization on body size in the spotted lanternfly (Lycorma delicatula), an invasive planthopper (Hemiptera: Fulgoridae) that is rapidly spreading across urban and non-urban settings in the United States. We then evaluated the consequences of spotted lanternfly body size for heat tolerance, a trait with importance for ectotherm survival in urban heat islands. Contrary to our expectations, we found that both male (P = 0.011) and female (P < 0.001) spotted lanternflies were larger in more urbanized areas and that females displayed a positive effect of body size on resistance to hot temperatures (P = 0.018). These results reject plasticity in developmental rate due to the urban heat island effect as an explanation for spotted lanternfly body size and instead lend necessary (but insufficient) support to an adaptive explanation stemming from advantages of larger body size in cities. This study demonstrates a positive effect of urbanization on spotted lanternfly body size, with potential implications for dispersal distance, fecundity, and thermal tolerance in urban areas.