Urban evapotranspiration (ET), a crucial component of the urban surface water-energy balance, is challenging to estimate accurately because most existing methods neglect the anthropogenic heat flux (AHF) generated by urbanization and human activities. This study quantified the global impact of AHF on ET estimation by comparing remote sensing-derived ET that includes and excludes AHF across 668 cities worldwide. On an annual scale, neglecting AHF resulted in significant ET underestimation, with absolute (relative) differences of 2.23 W/ m2 (7.9%) in cold regions, 2.56 W/m2 (9.8%) in Europe, and 2.81 W/m2 (11.0%) in megacities. In cold-region megacities, relative differences reached 16.0%, approximately 1.8 times higher than those in medium-sized cities in arid zones, and further increased to 9.70 W/m2 (32.7%) in compact high-rise and mid-rise building districts. Seasonally, the discrepancy peaked in summer (3.95 W/m2), nearly four times that in winter. During summer, cold-region megacities exhibited the largest differences (mean 8.49 W/m2), with maximum values up to 20.80 W/m2 in compact high-rise and mid-rise building districts. These findings provide the first global-scale quantitative evidence identifying the conditions under which the AHF should be considered in urban ET estimation. Moreover, incorporating the AHF improves the accuracy of ET retrieval and strengthens the assessment of urban water and energy budgets.