  We present a search for the lepton-flavor-violating decays $B{}^0_s
\rightarrow \ell^{\mp}\tau^{\pm}$, where $\ell = e, \mu$, using the full data
sample of $121~\mathrm{fb}^{-1}$ collected at the $\Upsilon(5S)$ resonance with
the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use
$B{}^0_s \overline{B}{}^0_s$ events in which one $B{}^0_s$ meson is
reconstructed in a semileptonic decay mode and the other in the signal mode. We
find no evidence for $B{}^0_s \rightarrow \ell^{\mp}\tau^{\pm}$ decays and set
upper limits on their branching fractions at $90\%$ confidence level as
$\mathcal{B}(B{}^0_s \rightarrow e^{\mp}\tau^{\pm}) < 14 \times 10^{-4}$ and
$\mathcal{B}(B{}^0_s \rightarrow \mu^{\mp}\tau^{\pm}) < 7.3 \times 10^{-4}$.
Our result represents the first upper limit on the $B{}^0_s \rightarrow
e^{\mp}\tau^{\pm}$ decay rate.
