Provided is an endoscope objective optical system that has a relatively narrow angle, a small outer diameter, and a short overall length and that allows simultaneous correction for field curvature and comatic aberration. An endoscope objective optical system (1) includes, in order from an object side, a first lens (2) composed of a negative single lens, a second lens (3) composed of a positive single lens, an aperture stop (4), a third lens (5) composed of a positive single lens, and a fourth lens (6) composed of a positive combined lens. The endoscope objective optical system (1) satisfies the following conditions: (1) 0.3 < Df/f < 1.15, (2) n1 < 1.79, (3) n2 > n1, and (4) 0.6 < IH/f < 0.83 wherein 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, n1 is the refractive index of the first lens (d-line), n2 is the refractive index of the second lens (d-line), and IH is the maximum image height of the entire system.
