We report a measurement of the CP-violation parameter sin2$ϕ_1$ 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 $J/ψ K_S^0$ 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 $J/ψ K_S^0$ yields, we determine sin2$ϕ_1$ = 0.57 $±$ 0.58(stat) $±$ 0.06(syst). The results are based on 121 fb$^{-1}$ of data recorded by the Belle detector at the KEKB $e^+ e^-$ collider. ; Comment: 6 pages, 3 figures (submitted to PRL)