In the present study, an extremophilic GH11 xylanase was stabilized by an engineered N-terminal disulphide bridge. The effect of the stabilization was then tested against high temperatures and in the presence of a biomass-dissolving ionic liquid, 1-ethyl-3-methylimidazolium acetate ([emim]OAc). The N-terminal disulfide bridge increased the half-life of a GH11 xylanase (XYNB) from the hyperthermophilic bacterium Dictyoglomus thermophilum by 10-fold at 100 °C. The apparent temperature optimum increased only by ∼5 °C, which is less than the corresponding increase in mesophilic (∼15 °C) and moderately thermophilic (∼10 °C) xylanases. The performance of the enzyme was increased significantly at 100–110 °C. The increasing concentration of [emim]OAc almost linearly increased the inactivation level of the enzyme activity and 25% [emim]OAc inactivated the enzyme almost fully. On the contrary, the apparent temperature optimum did not decrease to a similar extent, and the degree of denaturation of the enzyme was also much lower according to the residual activity assays. Also, 5% [emim]OAc largely counteracted the benefit obtained by the stabilizing disulfide bridge in the temperature-dependent activity assays, but not in the stability assays. Km was increased in the presence of [emim]OAc, indicating that [emim]OAc interfered the substrate–enzyme interactions. These results indicate that the effect of [emim]OAc is targeted more to the functioning of the enzyme than the basic stability of the hyperthermophilic GH11 xylanase.