  We report a measurement of the mixing-induced and direct $\textit{CP}$
violation parameters $S_{\text{CP}}$ and $A_{\text{CP}}$ from $B^0 \to J/\psi
K^0_S$ decays reconstructed by the Belle II experiment at the SuperKEKB
asymmetric-energy electron-positron collider. The data, collected at the
center-of-mass energy of the $\Upsilon(4S)$ resonance, correspond to
$190\text{fb}^{-1}$ of integrated luminosity. We measure ${S_{\text{CP}} =
0.720\pm0.062\pm0.016}$ and $A_{\text{CP}} = 0.094\pm0.044^{+0.042}_{-0.017}$,
where the first uncertainties are statistical and the second systematic. In the
Standard Model, $S_{\text{CP}}$ equals $\sin(2\phi_1)$ to a good approximation.
