  We report a measurement of decay-time dependent charge-parity ($CP$)
asymmetries in $B^0 \rightarrow K^0_S K^0_S K^0_S$ decays. We use $387 \times
10^6 B\bar{B}$ pairs collected at the $\Upsilon(4S)$ resonance with the Belle
II detector at the SuperKEKB asymmetric-energy electron-positron collider. We
reconstruct 220 signal events and extract the $CP$-violating parameters $S$ and
$C$ from a fit to the distribution of the decay-time difference between the two
$B$ mesons. The resulting confidence region is consistent with previous
measurements in $B^0 \rightarrow K^0_S K^0_S K^0_S$ and $B^0 \rightarrow
(c\bar{c})K^0$ decays, and with predictions based on the standard model.
