When applied to dynamical systems, both classical and quantum, time periodic modulations can produce complex nonequilibrium states which are often termed 'chaotic'. Being well understood within the unitary Hamiltonian framework, this phenomenon is less explored in open quantum systems. Here we consider quantum chaotic state emerging in a leaky cavity, when an intracavity photonic mode is coherently pumped with the intensity varying periodically in time. We show that a single spin, when placed inside the cavity and coupled to the mode, can moderate transitions between regular and chaotic regimes -that are identified by using quantum Lyapunov exponents -and thus can be used to control the degree of chaos. In an experiment, these transitions can be detected by analyzing photon emission statistics.A passage connecting Chaos Theory 1 and many-body quantum physics is provided by the mean-field ideology 2-4 and different semiclassical approximations 5,6 . They declare that, when the number N of quantum degrees of freedoms is systematically increased, the exponentially complex evolution of a quantum model can be approximated with a fixed size system of classical non-linear differential equations. These equations model the dynamics of the expectation values of relevant observables and the model becomes exact in the thermodynamic limit N → ∞. A degree of chaos in the original quantum system can be quantified by calculating standard classical quantifiers (usually maximal Lyapunov exponents 6,7 ) for the corresponding classical system. In the case of an open quantum system, the mean-field approach can be realized on the level of density matrices 8 . Alternatively, the adjoint form of a Markovian master equation, governing the evolution of the system density matrix 4 , can be employed [9][10][11] . What if we are dealing with an open model and do not want (or simply do not have a possibility) to go into the (semi)classical limit or resort to a mean-field description? When the evolution of the system is modeled with a master equations of the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) form 4 , a recently proposed idea of quantum Lyapunov exponents 12 provides a possibility to quantify the degree of chaos in a straightforward manner. Here we implement this idea and demonstrate how chaotic regimes of a system with N 1 states (a photonic mode in an open cavity) can be controlled by coupling it to a single spin.