The purpose of this work was to estimate the crop evapotranspiration (ETc) and to determine the crop coefficient (Kc) of Typha latifolia in constructed wetlands. The experiment used two horizontal subsurface flow constructed wetlands (HFCWs) cultivated with T. latifolia. Each HFCW received 50 L of secondary effluent daily, resulting in 3.2 days of nominal hydraulic retention time (disregarding effects of evapotranspiration and precipitation). In order to determine the ETc, water mass balance of the HFCWs was performed. In this analysis, ETc was considered equal to the difference between the inflow (secondary effluent + precipitation) and the outflow. The Kc was calculated using ETc and reference evapotranspiration (ETo) data (Kc = ETc/ETo). The results showed that 7-day average ETc varied dramatically within the range of 4.9–20.0 mm day−1, with a monthly average of 9.85 mm day−1. The mean monthly values of Kc varied from 2.03 to 3.68 (mean: 2.74). ETc is very important with respect to the HFCW water balance because it decreases effluent outflow considerably. Therefore, the findings of this work indicate that water losses caused by evapotranspiration should be considered when designing constructed wetlands.