A numerical approach is presented to assess the seismic vulnerability of barrel masonry vaults and evaluate the eectiveness of a traditional retrofitting intervention consisting in the reinforcement of the extrados. A linear elastic no–tension model is adopted to cope with the negligible strength in tension of ancient brick and stone masonry and perform a two–dimensional finite element analysis of arch–like sections. Instead of implementing conventional load history analysis or limit load analysis, the minimization of the relevant strain energy function is implemented to solve the non–linear equilibrium under the effect of dierent load scenarios. A segmental barrel vault made of stone masonry is investigated in an ancient building under static and seismic loads. The collapse load of the structural element is computed before and after the intervention and the reduction achieved in terms of seismic vulnerability is evaluated as prescribed by technical codes.