We study out-of-time-order correlators (OTOCs) of the form <(A) over cap (t) (B) over cap (0) (C) over cap (t) (D) over cap (0)> for a quantum system weakly coupled to a dissipative environment. Such an open system may serve as a model of, e. g., a small region in a disordered interacting medium coupled to the rest of this medium considered as an environment. We demonstrate that for a system with discrete energy levels the OTOC saturates exponentially alpha Sigma a(i)e(-t/tau i) + const to a constant value at t -> infinity, in contrast with quantum-chaotic systemswhich exhibit exponential growth of OTOCs. Focusing on the case of a two-level system, we calculate microscopically the decay times tau(i) and the value of the saturation constant. Because some OTOCs are immune to dephasing processes and some are not, such correlators may decay on two sets of parametrically different time scales related to inelastic transitions between the system levels and to pure dephasing processes, respectively. In the case of a classical environment, the evolution of the OTOC can be mapped onto the evolution of the density matrix of two systems coupled to the same dissipative environment.