An objective optical system for endoscope includes, in order from an object side, a front lens group having a negative refractive power as a whole, an aperture stop, and a rear lens group having a positive refractive power as a whole. The front lens group includes, in order from the object side, a single first lens having a negative refractive power and a single second lens having a positive refractive power. The rear lens group includes a single third lens having a positive refractive power, and a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power. An object-side surface of the first lens is a flat surface, the second lens has a meniscus shape having a convex surface directed toward an image side, and the third lens has a biconvex shape.