Ce0.8Sm0.2O2-δ (SDC) is one of the state-of-the-art oxide ion conductors, exhibiting high conductivity, and is considered to be the most promising electrolyte material for low-temperature solid oxide fuel cells (SOFCs). In this work, proton conduction behavior was demonstrated in this well-known oxide ion conductor at high temperatures by performing hydrogen permeation test. Dense SDC films were deposited on porous NiO–SDC substrates by a dry-pressing technique followed by co-firing at 1400°C for 5 h. Hydrogen permeability of the asymmetric micron-scale thick SDC films was evaluated for the first time. The dependence of the hydrogen permeation flux on film thickness, operating temperature and hydrogen chemical gradient was investigated. It was found that bulk diffusion is the rate-limiting factor for hydrogen permeation through the SDC film. The proton conductivity of the SDC film was also estimated according to Wagner equation, and varied from 3.03 × 10−6 to 1.09 × 10−5 S cm−1 at 950°C with the feed gas ranging from 20% H2/N2 to 80% H2/N2.