We report the first detection of the phosphorus monoxide ion (PO + ) in the interstellar medium. Our unbiased and very sensitive spectral survey toward the G+0.693–0.027 molecular cloud covers four different rotational transitions of this molecule, two of which ( J = 1–0 and J = 2–1) appear free of contamination from other species. The fit performed, assuming local thermodynamic equilibrium conditions, yields a column density of N =(6.0 ± 0.7) × 10 11  cm −2 . The resulting molecular abundance with respect to molecular hydrogen is 4.5 × 10 –12 . The column density of PO + normalized by the cosmic abundance of P is larger than those of NO + and SO + , normalized by N and S, by factors of 3.6 and 2.3, respectively. The N (PO + )/ N (PO) ratio is 0.12 ± 0.03, more than one order of magnitude higher than that of N (SO + )/ N (SO) and N (NO + )/ N (NO). These results indicate that P is more efficiently ionized than N and S in the ISM. We have performed new chemical models that confirm that the PO + abundance is strongly enhanced in shocked regions with high values of cosmic-ray ionization rates (10 –15 − 10 –14 s −1 ), as occurring in the G+0.693–0.027 molecular cloud. The shocks sputter the interstellar icy grain mantles, releasing into the gas phase most of their P content, mainly in the form of PH 3 , which is converted into atomic P, and then ionized efficiently by cosmic rays, forming P + . Further reactions with O 2 and OH produces PO + . The cosmic-ray ionization of PO might also contribute significantly, which would explain the high N (PO + )/ N (PO) ratio observed. The relatively high gas-phase abundance of PO + with respect to other P-bearing species stresses the relevance of this species in the interstellar chemistry of P.