  A measurement of the $B^0_s \to J/\psi\phi$ decay parameters using 80.5
$\mathrm{fb}^{-1}$ of integrated luminosity collected with the ATLAS detector
from 13 TeV proton-proton collisions at the LHC is presented. The measured
parameters include the $CP$-violating phase $\phi_s$, the width difference
$\Delta\Gamma_{s}$ between the $B^0_s$ meson mass eigenstates and the average
decay width $\Gamma_{s}$. The values measured for the physical parameters are
combined with those from 19.2 $\mathrm{fb}^{-1}$ of 7 TeV and 8 TeV data,
leading to the following: \begin{eqnarray*} \phi_s & = & -0.087\phantom{0} \pm
0.036\phantom{0} ~\mathrm{(stat.)} \pm 0.021\phantom{0} ~\mathrm{(syst.)~rad}
\\ \Delta\Gamma_{s} & = & \phantom{-}0.0657 \pm 0.0043 ~\mathrm{(stat.)} \pm
0.0037 ~\mathrm{(syst.)~ps}^{-1} \\ \Gamma_{s} & = & \phantom{-}0.6703 \pm
0.0014 ~\mathrm{(stat.)} \pm 0.0018 ~\mathrm{(syst.)~ps}^{-1} \\
\end{eqnarray*} Results for $\phi_s$ and $\Delta\Gamma_{s}$ are also presented
as 68% confidence level contours in the $\phi_s$-$\Delta\Gamma_{s}$ plane.
Furthermore, the transversity amplitudes and corresponding strong phases are
measured. $\phi_s$ and $\Delta\Gamma_{s}$ measurements are in agreement with
the Standard Model predictions.
