A natural generalization of the class of minimal submanifolds from the variational point of view is the one of biharmonic submanifolds (see, [9]), which was introduced by Eells and Sampson [9]. Thus, it is worthwhile and interesting to investigate biharmonic Lagrangian submanifolds. For recent developments in the study of biharmonic submanifolds see, for example, [2], [3], [10] and [15]. In this paper, biharmonic Lagrangian surfaces of constant mean curvature in complex space forms are classified. In particular, we obtain new examples of marginally trapped Lagrangian surfaces in an indefinite complex Euclidean plane. This implies that the classification of marginally trapped biharmonic surfaces due to Chen and Ishikawa [6] is not complete.