We report the observation of the exclusive decay B0s→D−sD+s at the 7.5 standard deviation level using 355  pb−1 of data collected by the CDF II detector in pp collisions at s√=1.96  TeV at the Fermilab Tevatron. We measure the relative branching ratio B(B0s→D−sD+s)/B(B0→D−D+s)=1.44+0.48−0.44. Using the world average value for B(B0→D−D+s), we find B(B0s→D−sD+s)=(9.4+4.4−4.2)×10−3. This provides a lower bound ΔΓCPs/Γs≥2B(B0s→D−sD+s)>1.2×10−2 at 95% C.L.