It is shown that in D = 3 space-time dimensions there exist a topological term for the bosonic strings. The corresponding constraints satisfy the same Virasoro algebra as the ordinary bosonic strings. These results are generalized for an arbitrary dimensional space-time if we have SO (1, 2) ⊗ O (D − 3) or SO (3) ⊗ O (1, D − 4) symmetry instead of SO (1, D − 1) space-time symmetry. A gauge-dependent correction to the Casimir energy corresponding to this topological term is derived.