As important natural features that deliver diverse ecosystem services including temperature regulation, water purification, and habitat provision, urban lakes are among the most revealing artefacts of contemporary urbanization. Once peripheral, they are now located within cities as waterfront amenities, leisure lakes, and neighborhood parks. This shift reflects not only changing urban form but also the human nature to seek out bodies of water for openness, reflection, and relief. It also highlights why urban lakes merit distinct attention: their enclosed and relatively still waters within urban spaces make them especially sensitive to human and environmental pressures. Yet their proliferation has outpaced our understanding. Across the world, cities continue to infill, dredge, and excavate waterbodies without a shared definition of what constitutes an “urban lake”. In China, for instance, our ongoing research has identified 16,522 urban lakes, but global hydrological datasets still treat them indistinguishably from other waterbodies. Despite their growing importance,1 urban lakes remain conceptually underdefined. This commentary addresses this gap by proposing an integrated framework that clarifies what constitutes an urban lake, synthesizes its ecological, social, and economic functions, and outlines future research directions for understanding them within rapidly urbanizing environments. [...]