We calculate the complete next-to-next-to-leading (NNLO) order QCD corrections to the charm contribution of the rare decay K⁺→π⁺ν. We present the results for the two-loop matching conditions of the Wilson coefficients, the three-loop anomalous dimensions, and the two-loop matrix elements of the relevant operators entering the NNLO renormalization group analysis of the Z-penguin and the electroweak box contribution. The NNLO QCD corrections reduce the theoretical uncertainty from ±9.8% at NLO to ±2.4% in the relevant parameter P_c(X), implying the leftover scale uncertainties in (K⁺→π⁺ν) and in the determination of |V_td|, sin 2β, and γ from the K→πν system to be ±1.3%, ±1.0%, ±0.006, and ±1.2°, respectively. For the charm quark mass m_c(m_c) = (1.30±0.05)GeV and |V_us| = 0.2248 the NLO value P_c(X) = 0.37±0.06 is modified to P_c(X) = 0.38±0.04 at NNLO and the error is fully dominated by the uncertainty in m_c(m_c). We tabulate P_c(X) in terms of m_c(m_c) and α_s(M_Z) and express the dependences of P_c(X) on these and other parameters by an accurate approximate analytic formula. We find (K⁺→π⁺ν) = (8.0±1.1) × 10⁻¹¹ and the quoted uncertainty mainly stems from m_c(m_c) and the Cabibbo-Kobayashi-Maskawa elements. We also emphasize that improved calculations of the long-distance contributions to K⁺→π⁺ν and of the isospin breaking in the weak current matrix element will further sharpen the sensitivity of the two golden K→πν decays to new physics.