This study aimed to evaluate the performance of a horizontal subsurface-flow constructed wetland (HSSF-CW) in reducing the microbial load of municipal wastewater and to assess the microbial quality of the effluent under the hot and semi-arid climatic conditions of Dezful, Iran. An experimental system consisting of four parallel units was evaluated, including an unplanted control unit, a monoculture of Phragmites australis, a monoculture of Chrysopogon zizanioides, and a mixed plantation, at three nominal hydraulic retention times (HRTs) of 5, 4, and 3 days. The microbial indicators, including total coliforms (TC) and fecal coliforms (FC), were measured in the influent and effluent, and system performance was evaluated based on removal efficiency and the apparent first-order removal rate coefficient (k). The results showed that the vegetated units, particularly the mixed plantation, achieved greater reductions in TC and FC concentrations than the unplanted control. The highest removal efficiency was observed in the mixed plantation at an HRT of 5 days, reaching 99.9% for both indicators. The k values ranged from 0.07 to 1.48 d⁻¹. Despite the substantial reduction in microbial load, the effluent concentrations of TC and FC did not comply with the referenced microbiological quality limits under any of the operating conditions. Therefore, although increasing the HRT and using vegetation improved coliform removal, these measures alone were insufficient to ensure the microbiological quality required for different discharge or water-reuse applications. Consequently, for applications with more stringent microbiological requirements, additional disinfection or integration of the HSSF-CW with complementary treatment units may be required.