Inspired by plant movements caused by water swelling and deswelling within nano- and mesoscale cellulose fibrillar structures, asymmetric exposure of water vapors to thin films of nanofibrillated cellulose, NFC (also denoted as microfibrillated cellulose, MFC, or native cellulose nanofi bers, CNF) is here shown to induce humidity-controlled reversible actuation. The extent of the bending depends on the humidity difference across the film. A steady-state bending is reached within a fraction of minute upon continued humidity exposure, and the film relaxes back upon removing the imposed humidity. The bending curvature becomes reduced when the film thickness increases, thus explaining that the effect is not observable in classic paper sheets, which are much thicker due to their constituent macroscopic thicker cellulose fibers. The bending is highly sensitive to humidity, as demonstrated by the observation that the film bends even based on the small humidity of human hand from a distance of several millimeters. Such a bending of a cellulose nanofiber film offers a particularly simple route toward biomimetic actuation and novel types of active material.