This study proposes a steel damper with a simple shape and excellent energy dissipation capacity. The proposed damper has a rectangular shape (R-type) and has an energy dissipation part and a load transmission part. The energy dissipation part dissipates external energy through the yielding of the steel material; it comprises a vertical member and upper and lower horizontal members. This study performed two-phase experiments to verify the structural performance of the proposed damper. The Phase I test was performed to evaluate the load history characteristics and energy dissipation capacity of the damper and the Phase II test was performed to confirm the structural performance of reinforced concrete members with the proposed damper. The experimental results showed that the proposed R-type steel damper had high-energy dissipation performance despite having a simple shape.