The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S-1 (1)(n(Se)pi(5)* and S-2 (1)(pi(Se)pi(5)*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T-1 (3)(pi(Se)pi(5)*) and T-2 (3)(pi(Se)pi(5)*), are found energetically below the S-2 state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S-2 state and destabilizes the S-1, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C=Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S-2 state, 6SeG evolves to the S-2 minimum, near to a S-2/S-1 conical intersection that allows for internal conversion to the S-1 state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S-2/S-1 conical intersection. From the S-1 state, 6SeG can further decay to the T-1 (3)(pi(Se)pi(5)*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T-1 minimum and the T-1/S-0 crossing point. Although this general S-2 -> T-1 mechanism takes place in both media, the presence of DNA induces a steeper S-2 potential energy surface, that it is expected to accelerate the S-2 -> S-1 internal conversion.