We report an observation of the decay B0s→D−sπ+ in pp collisions at s√=1.96  TeV using 115  pb−1 of data collected by the CDF II detector at the Fermilab Tevatron. We observe 83±11(stat) B0s→D−sπ+ candidates, representing a large increase in statistics over previous measurements and the first observation of this decay at a pp collider. We present the first measurement of the relative branching fraction B(B0s→D−sπ+)/B(B0→D−π+)=1.32±0.18(stat)±0.38(syst). We also measure B(B+→D0π+)/B(B0→D−π+)=1.97±0.10(stat)±0.21(syst), which is consistent with previous measurements