PROBLEM TO BE SOLVED: To provide an endoscope objective optical system which is compact and yet offers a wide field of view and high resolution.SOLUTION: An endoscope objective optical system includes, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. The first lens group comprises a plano-concave lens having a concave surface on the image side and a negative meniscus lens, and the second lens group comprises an object-side positive lens, an aperture stop, and an image-side positive lens, where the object-side positive lens is positioned to have a convex surface on the object side, the object-side surface having a curvature radius smaller than that of an image-side surface, and the image-side positive lens is positioned to have a convex surface on the image side, the image-side surface having a curvature radius smaller than that of an object-side surface. The third lens group includes at least one cemented lens. The endoscope objective optical system satisfies a conditional expression (1) below: 1.8
