Lavender essential oil (LT) is subject to adulteration by malicious suppliers and is often sold mixed with the cheaper lavandin essential oil (LD). In this study, it is aimed to develop a new and rapid spectroscopic method by using chemometric modeling techniques for the qualitative and quantitative determination of LD adulteration in LT. For the discrimination of LD in LT, 36 principal component analysis (PCA) models were created and the model with the best statistical results was evaluated by the number of PCs in the explained variance and eigen value. Soft independent modeling by class analogy (SIMCA) was also employed to demonstrate the discrimination capabilities of the best PCA model. For the quantification of LD in LT, 36 partial least squares (PLS) regression models employing the spectral data were developed. To find out the best PLS model, the root mean square error of prediction (RMSEP), and root mean square error of cross-validation (RMSECV) were mainly used. It was found that the best models for discrimination and quantitative analysis were those using normal spectra in the region 2200–400 cm−1 (NDSR). The results obtained showed that LD adulteration in LT could be successfully detected over a wide concentration range, thanks to the combined use of FTIR spectroscopy and chemometry without using any toxic chemicals or pretreatments.