A new interstellar molecule, KCN, has been identified toward the circumstellar envelope of the carbon-rich asymptotic giant branch star, IRC+10216—the fifth metal cyanide species to be detected in space. Fourteen rotational transitions of this T-shaped, asymmetric top were searched for in the frequency range of 83-250 GHz using the Arizona Radio Observatory (ARO) 12 m Kitt Peak antenna, the IRAM 30 m telescope, and the ARO Submillimeter Telescope. Distinct lines were measured for 10 of these transitions, including the Ka = 1 and 2 asymmetry components of the J = 11 → 10 and J = 10 → 9 transitions, i.e., the K-ladder structure distinct to an asymmetric top. These data are some of the most sensitive astronomical spectra at λ ~ 1 and 3 mm obtained to date, with 3σ noise levels ~0.3 mK, made possible by new ALMA technology. The line profiles from the ARO and IRAM telescopes are consistent with a shell-like distribution for KCN with r outer ~ 15'', but with an inner shell radius that extends into warmer gas. The column density for KCN in IRC+10216 was found to be N tot ≈ 1.0 × 1012 cm-2 with a rotational temperature of T rot ~ 53 K. The fractional abundance was calculated to be f(KCN/H2) ~ 6 × 10-10, comparable to that of KCl. The presence of KCN in IRC+10216, along with MgNC, MgCN, NaCN, and AlNC, suggests that cyanide/isocyanide species are the most common metal-containing molecules in carbon-rich circumstellar gas.