A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling particularly promising for the development of hybrid spin-mechanical systems and coherent mechanical spin driving. We demonstrate here that the spin of an individual Cr atom inserted in a semiconductor quantum dot can be controlled optically. We first show that a Cr spin can be prepared by resonant optical pumping. Monitoring the time dependence of the intensity of the resonant fluorescence of the quantum dot during this process permits to probe the dynamics of the optical initialization of the Cr spin. Using this initialization and read-out technique we measured a Cr spin relaxation time at T=5 K of about 2 microseconds. We finally demonstrate that, under a resonant single mode laser field, the energy of any spin state of an individual Cr atom can be independently tuned by using the optical Stark effect.Individual spins in semiconductor nano-structures are promising for the development of quantum information technologies 1-3 . Spins trapped in optically active quantum dots (QDs) have attracted strong interest since their coupling to light enables fast spin control and optical coherent control has been demonstrated for electron 4,5 and hole 6,7 spins confined in QDs. Thanks to their expected longer coherence time, localized spins on individual dopants in semiconductors are also a promissing media for storing quantum information. Optically active QDs containing individual or pairs of magnetic dopants have been realized both in II-VI 8-12 and III-V 13,14 semiconductors. In these systems, since the confined carriers and magnetic atom spins become strongly mixed, an optical excitation of the QD can affect the spin state of the atom offering possibilities for a control of the localized spin 15,16 . The variety of magnetic transition elements that can be incorporated in semiconductors gives a large choice of localized electronic spins, nuclear spins and orbital momentums with optical addressability 8,[17][18][19] . This approach opens a diversity of applications in quantum information and quantum sensing.Among these magnetic atoms, chromium (Cr) is of particular interest 19 . It incorporates in II-VI semiconductors as Cr 2+ carrying an electronic spin S=2 and an orbital momentum L=2. Moreover, most of Cr isotopes have no nuclear spin. This simplifies the spin level structure and its coherent dynamics 20 . With bi-axial strain, the ground state of the Cr is an orbital singlet with spin degeneracy of 5. The orbital momentum of the Cr connects its spin to the local strain through the modification of the crystal field and the spin-orbit coupling. This spin to strain coupling is more than two orders of magnitude larger than for elements without orbital momentum (NV centers in diamond 21 , Mn atoms in II-VI semiconductors 22 ). Cr is therefore a promising qubit for hybrid spin-mechanical systems in which the motion of a mechanical oscillator would be coherently coupled to the spin state of a single atom 21,23,24 . The ...