This study empirically investigates weather effects on Korean stock index returns and market volatility based on the GJR-GARCH-X model. We focus on the issue about whether the weather effect is associated with financial deregulation and foreign investors’ transactions, which should not be affected by the weather conditions in Seoul City. To explore the weather effects by controlling for foreign investors’ trading, we employ daily stock index data and weather indicators (cloud cover, precipitation, sunshine hours, snow, and humidity) during the period from 1981 to 2016. Our empirical results suggest that there is little evidence for weather effects on KOSPI and stock returns by industry listed in the Korea Stock Exchange even if Korean financial market deregulations in 1997 increases foreign investors’ trading, but there is some evidence for weather effects on market volatilities.