This method involves the development of a highly sensitive spectrophotometric procedure to estimate lanthanum in an aqueous medium. The procedure involved the treatment of lanthanum with Alizarine Red S as chromophor reagent in presence of neutral surfactant (Triton X-100). At pH4.5, a soluble red complex was produced and exhibited absorption peak with maximum absorption at 523nm. The calibration graph was linear and compatible to Beer's law over the range of concentration 0.125 to 10.0 µg.ml-1 with determination coefficient (R2=0.9994). The apparent molar absorptivity of the complex La-ARS was found to be 0.7222x104 l.mol-1.cm-1 which corresponds to the sensitivity of Sandell’s index of 0.0192 µg.cm-2. The stoichiometric of the complex was studied and it is equal to 1:2 (La: ARS). The values of LOD, LOQ and the range of recovery percentage were estimated and found to be 0.04182, 0.04434 µg.ml-1 and 96.1% to 103.5%, respectively. The precision (RSD) was also calculated and found to be in the range 0.469 to 1.612. The proposed procedure has been applied successfully for the determination of lanthanum in natural waters and urine sample.