An optical system, a stereoscopic imaging device, and an endoscope each provides a small size and capable of obtaining a stereoscopic image with a high resolution and a wide observation view angle. An optical system 1 includes, in order from an object side to an image plane side, a front group Gf including a first front group Gf1 arranged centering a first front central axis Cf1 and a second front group Gf2 arranged centering a second front group central axis Cf2 as a rotation symmetry axis extending parallel to the first front group central axis Cf1 and a back group Gb arranged centering a single back group central axis Cb. A central principal ray of a first light beam L1 that has passed through the first front group Gf1 and a central principal ray of a second light beam L2 that has passed through the second front group Gf2 do not cross each other from when they exit from the back group Gb to when they reach the image plane.
