An endoscope objective optical system includes, in order from an object side, a first lens composed of a negative single lens, a second lens composed of a positive single lens, an aperture stop, a third lens composed of a positive single lens, and a fourth lens composed of a positive combined lens, the third lens having a meniscus shape having a convex surface facing the image side. The endoscope objective optical system satisfies the following conditions: 0.3 n1,  (3) 0.6 1.8  (5) Df is the distance from a surface of the first lens facing the object side to the aperture stop, f is the focal length of the entire system, IH is the maximum image height of the entire system, and d1 is the center thickness of the third lens.