  We compute the next-to-next-to-leading order (NNLO) QCD corrections to the
decay $t(\uparrow) \to X_b +W^+$ of a polarized top quark. The spin-momentum
correlation in this quasi two-body decay is described by the polar angle
distribution $\mathrm{d}\Gamma/\mathrm{d}\cos\theta_P=\frac{\Gamma}{2}(1+P_t\,
\alpha_P\, \cos\theta_P)$ where $P_t$ is the polarization of the top quark and
$\alpha_P$ denotes the asymmetry parameter of the decay. For the latter we find
$\alpha^{\mathrm{NNLO}}_P=0.3792\pm 0.0037$.
