CP violation in charm has not yet been observed, although measurements of time-integrated CP asymmetries in $D^0 \to K^+K^-$ and $D^0 \to \pi^+\pi^- decays$ have reached a remarkable precision, O(0.1%). The $D^0 \to K_S^0 K_S^0$ decay is a promising discovery channel for CP violation in charm. A prediction based on Standard Model gives an upper limit for the CP asymmetry of 1.1% (C.L. 95%). Further enhancements could result from contributions from physics beyond the Standard Model. We present a measurement of the time-integrated CP asymmetry in prompt $D^0 \to K_S^0 K_S^0$ decays, performed using data collected with LHCb experiment in 2015 and 2016 at a 13 TeV pp center-of-mass energy (Run-2). This result improves the sensitivity obtained by LHCb in Run-1.