An objective optical system for endoscope includes a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture stop, a third lens having a positive refractive power, a fourth lens having a positive refractive power, a fifth lens having a negative refractive power, and a sixth lens having a positive refractive power. The second lens is a meniscus lens having a convex surface directed toward an image side and the third lens is a meniscus lens having a convex surface directed toward the image side. A cemented lens having a positive refractive power is formed by the fourth lens and the fifth lens. The sixth lens is cemented to an image sensor, and the following conditional expression (1′″) is satisfied: 1≤(r3f+r3r)/(r3f−r3r)≤5  (1′″).