The two N-terminal PDZ domains of postsynaptic density protein-95 (PDS-95 PDZ1 and PDZ2) are closely connected in tandem by a conserved peptide linker of five amino acids. The interdomain orientation between PDZ1 and PDZ2 of the ligand-free PDZ12 tandem is restrained, and this conformational arrangement facilitates the synergistic binding of PDZ1 2 to multimeric targets.' The interdomain orientation of the target-bound state of PDZ12 is not known. Here, we have solved the structure of PDZ1 2 in complex with its binding domain from cypin. Both chemical shift data and residual dipolar coupling measurements showed that the restrained interdomain orientation disappeared upon cypin peptide binding. NMR-based relaxation experiments revealed slow interdomain motions in the PDZ12/cypin peptide complex. Molecular dynamics simulations also showed that the PDZ12/cypin complex has larger conformational flexibility than the ligand-free PDZ12. This dramatic change of protein dynamics provides extra conformational entropy upon ligand binding, thus enhancing the ligand binding affinity of the PDZ1 2 tandem. Modulation of ligand binding affinity through concerted interdomain structural and dynamic rearrangements may represent a general property of multidomain scaffold proteins.