  A measurement of the $CP$ asymmetry in $D^0 \to K^0_S K^0_S$ decays is
reported, based on a data sample of proton-proton collisions collected by the
LHCb experiment from 2015 to 2018, corresponding to an integrated luminosity of
6 fb$^{-1}$. The flavor of the $D^0$ candidate is determined using the charge
of the $D^{*\pm}$ meson, from which the decay is required to originate. The
$D^0 \to K^+ K^-$ decay is used as a calibration channel. The time-integrated
$CP$ asymmetry for the $D^0 \to K^0_S K^0_S$ mode is measured to be:
$$A^{CP}(D^0 \to K^0_S K^0_S) = (-3.1\pm 1.2\pm 0.4 \pm 0.2)\%, $$ where the
first uncertainty is statistical, the second is systematic, and the third is
due to the uncertainty on the $CP$ asymmetry of the calibration channel. This
is the most precise determination of this quantity to date.
