In this article, the authors probe the role of irrational investor sentiment in the determination of Indian stock market volatility. The authors developed a new irrational aggregate sentiment index (IASI) to examine the issue. The conditional volatility is extracted from the nonlinear univariate models for the market indices and the IASI. The vector autoregression (VAR) is carried out to analyze the relationship between the volatility of irrational aggregate sentiment index and stock market volatility. The authors find a unidirectional causality from sentiment to stock market volatility, and their findings highlight the significance of sentiment in explaining the stock market volatility in India.