The Local Meteoric Water Line (LMWL) is the relationship between the hydrogen and oxygen isotopes (δ2H and δ18O respectively) in precipitation and is the reference point for isotope investigations at a local area. The International Atomic Energy Agency (IAEA) have been collecting the Global Network of Isotopes in Precipitation (GNIP) since 1961. The GNIP is the primary source of δ2H and δ18O data in precipitation globally. The GNIP data gives critical information about the water cycle and enables a variety of hydrological processes and impacts to be studied. However, in most developing countries there is no (or limited) active stations where isotope data samples can be collected. For example, in Iran, the most recent rainwater isotope data was collected more than a decade ago in 2004. Therefore, an update of δ2H and δ18O and the LMWL is urgently needed for Iran, especially for Tehran, which is a rapidly growing urban center within Iran. In this paper, an update of Tehran LMWL is published. This update is based on one-year of data collected in 2014 and 2015 from Tehran’s rainwater and analyzed by IAEA. Regression results for the 32 samples yielded a LMWL defined by the equation LMWL (δD) = 7.83 δ18O+5.23 (r2 = 97%) based on ordinary least squares regression (OLSR) method and δD=8.18 δ18O+10.60 (r2 = 97%) based on precipitation amount weighted least squares regression (PWLSR) method. The updated LMWL equation presented here will be useful for future studies in this area that use stable isotopes to determine surface-water and groundwater interaction and sources of groundwater recharge.