Provided is an objective optical system for endoscopes wherein astigmatism and field curvature, along with distortion, are corrected in a satisfactory way. The objective optical system for the endoscopes includes, in the order from an object side, a first group G1 having a negative refractive power, an aperture diaphragm S, and a second group G2 having a positive refractive power. The first group G1 includes, in the order from the object side, a negative meniscus lens L1 with the convex surface facing the object and a positive lens L2 with the convex surface facing the object. The second group G2 includes, in the order from the object side, a prescribed lens L3 and two positive lenses. The negative meniscus lens L1 has an aspheric surface. The prescribed lens L3 is a meniscus lens with the convex surface facing an image. At least one of the two positive lenses is a cemented lens comprising a positive lens and a negative lens. The objective optical system satisfies the following conditional expressions (1) and (2):0.5
