In this paper we define and characterize cointegrated continuous-time linear state-space models. A main result is that a cointegrated continuous-time linear state-space model can be represented as a sum of a Lévy process and a stationary linear state-space model. Moreover, we prove that the class of cointegrated multivariate Lévy-driven autoregressive moving-average (MCARMA) processes, the continuous-time analogues of the classical vector ARMA processes, is equivalent to the class of cointegrated continuous-time linear state space models. Necessary and sufficient conditions for MCARMA processes to be cointegrated are given as well extending the results of Comte for MCAR processes. The conditions depend on the autoregressive polynomial. Finally, we investigate cointegrated continuous-time linear state-space models observed on a discrete time-grid and derive an error correction form for this model. The error correction form is based on an infinite linear filter in contrast to the finite linear filter for VAR models.