Cobalt sulfides have become promising electrode materials for lithium ion batteries while their applications in rechargeable magnesium batteries are rarely reported. In this paper, we have done some research on the electrochemical properties of cobalt sulfide (CoS) as the cathode-active material for rechargeable magnesium batteries. Flower-like CoS with nanostructures is synthesized by a facile solvothermal route. The obvious redox peaks on the cyclic voltammetric curves confirm the possibility of applications. The galvanostatic charge–discharge tests display excellent cycle stability and high coulomb efficiency. Meanwhile, the possible mechanism of charge–discharge reactions is proposed and discussed. These results show that flower-like CoS is a promising candidate as cathode-active material for rechargeable magnesium batteries.