In this paper we analyze the characteristic shape of the liquid meniscus at the fluid air interface in nanochannels of less than 80 nm height capped by a flexible membrane. Because of the induced negative pressure difference between the liquid pressure and the pressure outside, the 0.18 μm thin membrane on top of the channels bends downward. This elastocapillary equilibrium between the surface tension of the wetting liquid and the mechanical forces in the capillary results in a very peculiar shape of the interfacial meniscus, visible from the top through the transparent membrane. For increasing deflection of the membrane, the meniscus is seen to protrude along the channel and its curvature changes from concave to convex in the center. We present an analytical model to describe the meniscus shape in the deformed channel for small membrane deflections. We also show that the protrusion length of the meniscus, which can be measured easily, is an accurate and useful indicator for the membrane deflection. Experimental results on nanochannels filled with ethanol and water are presented and the observed menisci are seen to be in good agreement with the proposed model.