An optical system, a lens module, and an electronic device are provide. The optical system includes, in order from an object side to an image side along an optical axis, a first to seventh lenses. Each of the first lens, the third lens, and the sixth lens has a positive refractive power. Each of the second lens and the seventh lens has a negative refractive power. Each of the second lens, the third lens, and the sixth lens has an object-side surface which is convex near the optical axis. Each of the second lens and the seventh lens has an image-side surface which is concave near the oprtical axis. The third lens has an image-side surface which is convex near a periphery of the image-side surface of the third lens. The seventh lens has an object-side surface which is convex near the optical axis.