Provided is an endoscope objective optical system that is constituted of, in order from the object side, a positive first group, an aperture stop, and a positive second group, wherein the first group is constituted of a negative first lens whose surface on the object side is flat and a positive second lens; the second group is constituted of a combined lens formed of a positive third lens and a negative fourth lens; and Conditional Expressions (1) to (3) are satisfied. F31, F32, and f are the focal lengths of the third lens, the fourth lens, and the entire system, respectively; and R3 and R4 are the radii of curvature at the object-side surface and the image-side surface of the second lens, respectively. 1.2