A spacer includes a first surface and a second surface. The first surface and the second surface respectively have two opposite first roller accommodating grooves and two opposite second roller accommodating grooves. The first and second roller accommodating grooves of the first surface correspond to the second and first roller accommodating grooves of the second surface. The spacer defines a reference plane, which is perpendicular to a horizontal direction and passes through a center point of the maximum thickness of the spacer. The horizontal distance from the reference plane to the center of each first roller accommodating groove is different from the horizontal distance from the reference plane to the center of each second roller accommodating groove. By flipping the spacer of the present invention, the spacing of two adjacent rollers can be adjusted. In addition, the present invention further provides a cycloidal reducer with the aforementioned spacer.