The aim of this study is to investigate the effects of gadolinium addition on the microstructure and mechanical properties of duplex stainless steel (DSS) fabricated using a normal casting method. The oxygen content in the cast DSS alloy with gadolinium decreased because of the high reactivity of gadolinium with oxygen. The area fraction and size of non-intermetallic inclusions in the alloy decreased from 0.80±0.12% to 0.58±0.04% and from 6.9±0.7 to 5.8±0.4 μm upon gadolinium addition, respectively. Notably, the ultimate tensile strength and strain at break of the cast alloy significantly increased with the addition of gadolinium from 919±25 to 969±8 MPa and from 24.8±1.9% to 28.4±1.1%, respectively. The hardness of the cast alloy with gadolinium increased from 23.6±1.3 to 25.0±1.2 HRC. A significant increase in the impact energy of the cast alloy was observed and the brittle-to-ductile transition temperature slightly decreased by approximately 10 °C with the addition of gadolinium.