We report a measurement of the CP-violation parameter sin2φ 1 at the Υ(5S) resonance using a new tagging method, called "B-π tagging." In Υ(5S) decays containing a neutral B meson, a charged B, and a charged pion, the neutral B is reconstructed in the J/ψKS0 CP-eigenstate decay channel. The initial flavor of the neutral B meson at the moment of the Υ(5S) decay is opposite to that of the charged B and may thus be inferred from the charge of the pion without reconstructing the charged B. From the asymmetry between B-π + and B-π - tagged J/ψKS0 yields, we determine sin2φ 1=0.57±0.58(stat) ±0.06(syst). The results are based on 121fb -1 of data recorded by the Belle detector at the KEKB e +e - collider.