Metal dampers installed in base isolation layers are effective to control the displacement of the base isolation layers. The base-isolation layers deform in multiple directions during an earthquake, requiring that the dampers should also have multi-directional deformation capacity. Previous studies suggest that the U-shaped steel dampers have satisfactory deformation and energy dissipation capacities in-plane, and can significantly increase the damping of the structures and control the displacement of the base isolation layers. However, the out-of-plane behaviours of the U-shaped steel dampers are not necessarily satisfactory. To this end, a new slotted U-shaped steel damper is firstly proposed to improve the out-of-plane performances of the U-shaped steel damper. The effects of the damper width, height, and length on the damper performances are investigated through quasi-static tests. The test results suggest that the U-shaped steel damper with appropriate design parameters has satisfactory energy dissipation capacity and low cycle fatigue behaviour. The formulation is accurate for the estimation of the damper strength. Parametric finite element analysis is conducted to supplement the test results. Based on the results of parametric analysis, the formulas for yield displacement and restoring force of the slotted U-shaped damper are derived through regression analysis. The formulation to estimate the dampers ultimate restoring forces is also derived based on the virtual work principle. The accuracies and application conditions of the formulas are investigated according to the results of experiment and numerical analysis.