Municipal and industrial wastewater discharge is a longstanding environmental problem that pollutes water bodies, affecting both the landscape and human health. In the department of Sucre, Colombia, nearby urban sewage is discharged into the Arroyo Grande de la Sabana, and only Sincelejo city has a treatment system in place. Therefore, it is critical to identify effective treatment methods for removing contaminants from water. The objective of this study was to evaluate the efficiency of a constructed wetland (CW) with horizontal subsurface flow (HSSF) planted with Thalia geniculata for treating wastewater from the Arroyo Grande de la Sabana in Sucre, Colombia. The study investigated the effectiveness of a constructed wetland planted with Thalia geniculata for treating wastewater from the Arroyo Grande de la Sabana in Sucre, Colombia. Two different hydraulic retention times (HRTs) of 3 and 5 days were tested, and the plant population density was analyzed to determine the better adaptation of plants to the constructed wetlands. The results showed that on the fifth day of treatment, nitrate concentrations decreased by 33.22%, nitrite by 93.04%, and phosphate by 95.66%. Additionally, the biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) values decreased by 97.27% and 80.27%, respectively. On the third day of retention, turbidity, and total suspended solids (TSS) in the water decreased by 90.13% and 83.08%, respectively. The study concluded that the 5-day HRT was most effective in removing contaminants such as nitrites, nitrates, and phosphates, while the 3-day HRT was more efficient for TSS, turbidity, total coliforms (TCs), and fecal coliforms (FCs).