MoS2 is a promising material to replace the Pt catalyst in the electrochemical hydrogen evolution reaction (HER). It is well known that the activity of the MoS2 catalyst in the HER is significantly promoted by doping cobalt atoms. Recently, the Co-Mo-S phase, in which cobalt atoms decorate the edge positions of the MoS2 slabs, has been identified as a co-catalytic phase in the Co-doped MoS2 (Co-MoSx) with low Co content. Here, we report the effect of the incorporation of cobalt atoms in the chemical state of the Co-MoSx catalyst, which gives rise to the co-catalytic effect. Co-MoSx catalysts with various Co contents were prepared on carbon fiber paper by a simple hydrothermal process. On the Co-MoSx catalyst with high Co content (Co/Mo ≈ 2.3), a dramatically higher catalytic activity was observed compared to that for the catalyst with low Co content (Co/Mo ≈ 0.36). Furthermore, the co-catalytic phase in the Co-MoSx catalyst with the high Co content was found not to be the Co-Mo-S phase but was identified as CoS2 by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. It is believed that CoS2 is an alternative choice to co-catalyze HER on MoS2-based catalysts.