In forests where the overstorey canopy has been disturbed by timber harvest or other means, evapotranspiration (ET) by the understorey may be the main flux of water back to the atmosphere. However, little research has been apportioned to directly measuring understorey water use, and the technology to do so is thus limited. Here, we used a portable chamber to quickly and directly measure understorey ET by measuring the water flux from the soil and vegetation enclosed by the chamber. This method was used in a third‐growth coast redwood forest to measure ET rates from individuals of five species commonly found in the understorey: Sequoia sempervirens (coast redwood) sprouts, Notholithocarpus densiflorus (tanoak) sprouts, Vaccinium ovatum (evergreen huckleberry) sprouts, Polystichum munitum (sword fern) and Struthiopteris spicant (deer fern). Measurements were conducted in four small watersheds which were harvested to varying intensities ranging from 0% to 75% targeted reduction in preharvest overstory basal area. Mean understorey ET rate was highest in the watershed with the lowest residual basal area (x̅ = 0.036 ± 0.024 mm/h) and lowest in the unharvested control watershed (x̅ = 0.013 ± 0.0078 mm/h). Multiple regression modelling on resprouting species indicated that the difference in ET rate between watersheds was related to soil and light resources. These results imply that understorey water use can be significant, particularly in harvested watersheds, and should be quantified at the landscape scale.