The low-temperature state of CeRu2Al10 has been studied by neutron powder diffraction and muon spin relaxation. By combining both techniques, we prove that the transition occurring at T*similar to 27 K, whose origin has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with a propagation vector of k=(1, 0, 0) involves a collinear antiferromagnetic alignment of the Ce moments along the c axis of the Cmcm space group with a reduced moment of 0.34(2) mu(B). No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around T-2 similar to 12 K indicating a possible second transition.