Monolayer tungsten disulfide (WS2) has emerged as a material for optoelectronic applications because of its remarkable quantum yield of photoluminescence. However, the existing studies of defects in monolayer WS2 are insufficient to specifically discern Raman scattering properties caused by the defect. Here, we report that resonance tip-enhanced Raman spectroscopy imaging and correlation study with scanning tunneling microscopy can reveal defect-induced Raman modes denoted as D and D′ modes in monolayer WS2. Furthermore, our density functional theory calculations demonstrate that sulfur vacancies introduce not only the red-shifted A1g mode but also the D and D′ modes. The observed defect-related Raman modes can be utilized to evaluate the quality of monolayer WS2 and will be helpful to improve the performance of WS2 optoelectronic devices.