An imaging optical system includes lens group Gm located on that is closest to an object among lens groups in which a distance between the lens groups changes during zooming, the lens groups having negative power. Lens group Gm includes, in order from the object side toward an image side, lens element LGmF1 having the negative power, lens element LGmF2 having the negative power, both surfaces of lens element LGmF2 having an aspherical shape, and at least two lens elements having power. The present disclosure provides the imaging optical system having good various aberrations such as spherical aberration, astigmatism, and distortion and an imaging device and a camera system that are provided with the imaging optical system.