In proton exchange membrane fuel cells (PEMFC), the management of the oxygen excess ratio is crucial. Indeed, an insufficient amount of oxygen can lead to the problem of degradation of the performance and lifetime of the cell. However, this ratio is not measurable, hence the need to use an observer to estimate it. Unlike a standard high-gain observer, where measurement noise affects the observer performances, in this paper, we propose a filtered high-gain observer based on measurable variables such as compressor speed and supply manifold pressure. Thus, it is equipped with low-pass filter dynamics to reduce sensitivity to measurement noise. The stability of the observer is analyzed using Lyapunov stability tools. The simulation results highlight the superiority of the new proposed observer compared with a standard high-gain observer.