  We present the first complete high-precision QCD corrections to the inclusive
decay width $\mathrm{\Gamma}_t$, the $W$-helicity fractions
$f_{\mathrm{L,R,0}}$ and semi-inclusive distributions for the top-quark decay
process $t \rightarrow b + W^+ + X_{\mathrm{\tiny QCD}}$ at NNNLO in the strong
coupling constant $\alpha_s$. In particular, the pure NNNLO QCD correction
decreases the $\mathrm{\Gamma}_t$ by about $0.8\%$ of the previous NNLO result
at the top-quark pole mass scale, exceeding the error estimated by the usual
scale-variation prescription. After taking into account all sources of errors,
we get $\mathrm{\Gamma}_t = 1.3148^{+0.003}_{-0.005} + 0.027\,(m_t -
172.69)\,\text{GeV} $, the error of which meets the request by future
colliders. On the other hand, the NNNLO QCD effects on $f_{\mathrm{L,R,0}}$ are
found to be much smaller, at the level of one per-mille for the dominating
$f_{0}$, predestining them to act as precision observables for the top-quark
decay process.
