Provided is a high-performance objective optical system for an endoscope, which is compact, provides sufficient observation depth, has a wide viewing angle, and facilitates the observation and diagnosis of a lesion. This objective optical system for an endoscope comprises, in order from the object side: a front group FL having negative refractive power; an aperture stop S; and a rear group RL having positive refractive power. The front group FL has, in order from the object side: a first lens L1 comprising a negative single lens; and a second lens L2 comprising a positive single lens. The rear group RL has: a third lens L3 comprising a positive single lens; a cemented lens CL1 consisting of a positive fourth lens L4 and a negative fifth lens L5; and a positive sixth lens L6. The objective-side surface of the first lens L1 is a flat surface, the second lens L2 has a meniscus shape having a convex surface facing the image side, the sixth lens L6 is joined to an image capturing element IMG, and the following conditional expression (1) is satisfied:
-1.2 ≤D6/F12 ≤-0.47 ... (1),
where D6 is the thickness of the sixth lens L6, and F12 is the composite focal length from the first lens L1 to the second lens L2.