In the search for superconductivity in a BaAu2Sb2-type monoclinic structure, we have successfully synthesized the new compound BaPt2Bi2, which crystallizes in the space group P2(1)/m (No. 11; Pearson symbol mP10) according to a combination of powder and single crystal X-ray diffraction and scanning electron microscopy. A sharp electrical resistivity drop and large diamagnetic magnetization below 2.0 K indicates it owns superconducting ground state. This makes BaPt2Bi2 the first reported superconductor in a monoclinic BaAu2Sb2-type structure, a previously unappreciated structure for superconductivity. First-principles calculations considering spin orbit coupling indicate that Pt-Bi antibonding interaction plays a critical role in inducing superconductivity.