Poly(vinyl chloride) (PVC) is today one of the most important commodity polymers. Its broad range of applications is due to the presence of plasticizers whose concentration largely impacts the microscopic and the macroscopic properties. Quantifying the concentration of plasticizer in PVC products is therefore of fundamental importance. Thus, in this paper, the applicability of terahertz (THz) time-domain spectroscopy for the characterization of plasticized PVC is for the first time evaluated in a systematic way. It could be demonstrated that the method is able to distinguish between PVC samples with different types and concentrations of plasticizers. Furthermore, a simple, fast, and efficient method is introduced to quantify the concentration of plasticizer in PVC samples of known plasticizer type but different thermal histories. The presented results are of key importance due to the need of reliable noninvasive and nondestructive analytical methods which can deliver onsite information about the remaining plasticizer concentration inside PVC products. Furthermore, it is expected that the proposed approach can be easily extended to other plasticized polymers.