Provided is a small endoscope objective optical system that allows stereoscopic viewing even though a wide angle is used, that reduces the F-number, and that also reduces vignetting.Provided is an endoscope objective optical system that consists of, sequentially from the object side, a negative first lens group, a positive second lens group, and a pair of third lens groups that are disposed side-by-side in the direction of parallax and that form two optical images with parallax, wherein aperture stops are individually provided in the third lens groups, and conditional expression M below is satisfied: Fob_n/Fob_p>−0.5   (1), where Fob_n is a focal length of the first lens group, and Fob_p is a focal length of the second lens group.