We report an indication that the elastic scattering rate of solar $^8$B neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through the Earth during nighttime. We determine the day/night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be $(-3.2pm1.1(text{stat})pm0.5(text{syst}))%$, which deviates from zero by 2.7 $sigma$. Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a non-zero day/night asymmetry implies that the flavor oscillations of solar neutrinos are affected by the presence of matter within the neutrinos flight path. Super-Kamiokandes day/night asymmetry is consistent with neutrino oscillations for $4times10^{-5}$eV$^2leqDelta m^2_{21}leq7times10^{-5}$eV$^2$ and large mixing values of $theta_{12}$, at the $68%$ C.L.
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