Efficient hydraulic performance is critical for Constructed Wetlands (CWs), since poor hydraulic performance can reduce treatment efficiency by altering the actual residence time relative to the design value. However, no quantitative studies on the relationship between Hydraulic Efficiency (HE) and treatment efficiency in CWs have been reported to date. This study assessed the extent to which baffled-induced improvements in HE translated into improvements in pollutant removal efficiency. Five different baffle configurations were applied over a five-month period in a pilot-scale horizontal subsurface flow constructed wetland (HSFCW), and treatment efficiencies for Total Organic Carbon (TOC), Total Nitrogen (TN), and Total Phosphorus (TP) were monitored under varying HE conditions. The removal efficiency of TOC increased from 54.09% in a HSFCW with no baffles (HE = 0.5937) to 74.54% in a 7-baffled HSFCW (HE = 0.9520), similar results were observed for TN (from 41.42% to 62.39%), and TP (from 46.84% to 59.46%). A positive correlation was observed between HE and pollutant removal efficiency, where a higher HE was found to significantly enhance removal efficiency. Regression analysis demonstrated the quantitative relationship between HE and the removal efficiency of TOC (R