The B+/-_c meson is observed through the decay B+/-_c -->
J/psi pi++/-, in data corresponding to an integrated luminosity of 2.4
fb-1 recorded by the CDF II detector at the Fermilab Tevatron. A signal
of 108 +/- 15 candidates is observed, with a significance that exceeds
8 sigma. The mass of the B+/-_c meson is measured to be 6275.6 +/- 2.9
(stat.) +/- 2.5 (syst.) MeV/c2.