There is provided an endoscope objective optical system with a small size and a high image quality, and in which, an adequate space for disposing a visual-field direction converting element is secured. The endoscope objective optical system includes in order from an object side, a first group G1 having a negative refractive power, an aperture stop S, and a second group G2 having a positive refractive power, and the first group G1 includes in order from the object side a first lens L1 having a negative refractive power and a visual-field direction converting member P, and the second group G2 includes in order from the object side, a second lens L2 having a biconvex shape, and a cemented lens in which a third lens L3 having a positive refractive power and a fourth lens L4 having a negative refractive power are cemented, and the endoscope objective optical system satisfies the following conditional expressions (1), (2), and (3). 2.4≤d1/f≤4.6 1.85≤f2/f≤2.6 −50≤r21/r22≤−0.4 where,d1 denotes an air conversion length from a surface on an image side of the first lens up to a surface of the aperture stop,f denotes a focal length of the overall endoscope objective optical system,f2 denotes a focal length of the second group,r21 denotes a radius of curvature of a surface on the object side of the second lens, andr22 denotes a radius of curvature of a surface on the image side of the second lens.
