In this article, quantum-dot semiconductor optical amplifiers (QD-SOAs) have been modelled using state space method. To derive this model, we have manipulated the rate equation model of the QD-SOA, where the average values of the occupation probabilities along the QD-SOA cavity are considered as the state variables of the system. Using these variables, the distance dependence of the rate equations is eliminated. The derived state space model gives the optical gain and output signal of the amplifier with a high accuracy. Simulation results show that the derived model is not only much simpler and faster than conventional rate equation models, but also the optical gain and output signal of the investigated QD-SOA are calculated with a higher precision compared to the rate equation model. Copyright © 2013 John Wiley & Sons, Ltd.