An oxide semiconductor device includes a substrate, a first patterned oxide semiconductor layer, a source electrode, a drain electrode, and a sidewall spacer. The first patterned oxide semiconductor layer is disposed on the substrate. The source electrode and the drain electrode are disposed on the first patterned oxide semiconductor layer. The sidewall spacer is disposed on a sidewall of the first patterned oxide semiconductor layer. The sidewall spacer may be used to improve the performance of blocking impurities from entering the first patterned oxide semiconductor layer via the sidewall, and the electrical performance and the reliability of the oxide semiconductor device may be enhanced accordingly.