We have measured the ^12^C/^13^C interstellar ratio in the Coalsack, a prominent dark nebula in the southern sky, by means of high resolution (λ/δλ=6x10^4^) observations of the λ4232A CH^+^ line in the direction of the star HD110432 (d=~0.31kpc). The collection of a total of 53 spectra of this star and of a reference bright star (α Vir) has allowed us to obtain a mean spectrum with signal-to-noise ratio =~1500 at 1 σ level. We have detected the weak ^13^CH^+^ absorption well resolved from the ^12^CH^+^ line and derived a column density ratio =71+/-11 by means of line profile fitting with Voigt profiles. This result is in very good agreement with the few existing determinations of the column density ratio in other 4 clouds at d <~0.5kpc. The isotopic ratio is remarkably constant along different directions of the local interstellar medium and has a mean value =67+/-3. This value is in good agreement with the most recent estimates of the ^12^C/^13^C ratio in the solar vicinity derived from CO and H_2_CO emission lines observed at radio frequencies.