A novel, simple, rapid and stability-indicating reversed-phase ultra performance liquid chromatographic and mass spectrometric method was developed and subsequently validated for quantitation of Efavirenz (EFV) from drug substance matrix. The separation was achieved in 2.5 minutes on Waters ACQUITY UPLC BEH C18 (50 x 2.1) mm, 1.7µm column in isocratic mode with flow rate 0.4 mL/min. Mobile phase used was 0.01 M ammonium acetate buffer pH 7.5 and acetonitrile in ratio 50:50 v/v. Detection was carried out at the maximum wavelength of 247 nm using a photodiode array detector. The retention time of Efavirenz was found 1.8 minutes. Specificity of the method was established on drug substance by hydrolytic and oxidative stress conditions. Validation of analytical method was carried out as per the current ICH guidelines for linearity, recovery, precision, limit of detection, limit of quantification and robustness parameters.