A survey was performed to trace the main source of anthropogenic Pb pollution in Mexico City through Pb isotopic signatures ( 208 Pb/ 204 Pb, 206 Pb/ 204 Pb, 206 Pb/ 207 Pb, and 208 Pb/ 207 Pb) from 103 urban topsoil (0–5cm) samples. Those were collected in the metropolitan area of Mexico City and compared with isotopic compositions of leaded gasoline (LG), domestic Pb ores (DLO) and parent rock (PR). The isotope ratios (IRs) of Pb were determined by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) and total Pb concentration analyzed by wavelength dispersive X-ray fluorescence (WDXRF). The range of Pb concentrations levels in urban topsoil samples was 15–473mg/kg. The IR values obtained for these samples were 37.965–39.718 ( 208 Pb/ 204 Pb), 18.375–19.204 ( 206 Pb/ 204 Pb), 1.177–1.218 ( 206 Pb/ 207 Pb) and 2.443–2.496 ( 208 Pb/ 207 Pb). Analyzed topsoil samples with low Pb content (<50mg/kg) displayed high dispersion in 208 Pb/ 204 Pb values, which are determined by different natural sources. Samples with 51–200mg/kg Pb content, shown low dispersion that revealed the mixing between the natural Pb and anthropogenic Pb. The assessment of the IR values shown that, as Pb concentration increases, a trend toward gasoline IR data has been observed. The results obtained by this research suggest that although the use of leaded petrol had been banned in Mexico since 1997, the Pb pollution in the urban topsoils due to the historical use of Pb in petrol is still significant.