In this paper, the terahertz near-field response of monolayer MoS₂ and WS₂ grains prepared by chemical vapor deposition was investigated by terahertz scattering scanning near-field optical microscopy (THz s-SNOM). No resolvable terahertz near-field response was detected in the absence of visible excitation, indicating that the grains have a low doped carrier concentration. With visible light excitation, we were able to measure a terahertz near-field micrograph that exactly matches the grain profile due to the terahertz near-field response of the photogenerated carriers. Under the same photoexcitation conditions, the terahertz near-field response of MoS₂ is stronger than that of WS₂, it reflects the difference of carrier concentration or mobility between them. The results show that THz s-SNOM combines ultra-high spatial resolution and sensitive detection of photogenerated carriers. It is uniquely suited for micromechanics studies of the optoelectronic properties of two-dimensional semiconductor materials and devices.