Recent research on liquidity has reported that aggregate liquidity in the stock market varies over time, and evidence suggests that this variation affects stock returns. Although the importance of market liquidity in asset pricing has been well documented, little is known about what causes stock market liquidity to vary over time. This paper examines whether the time-series variation in stock market liquidity is related to investor sentiment. Using the liquidity measure developed by Amihud [2002] and two survey-based investor sentiment indices, I find that the stock market is more liquid when sentiment indices rise, that is, when investors are more bullish. Moreover, the Granger causality tests suggest that investor sentiment Granger-causes market liquidity. Further analyses show that market trading volume also increases when investor sentiment is higher. In addition, the finding that the market is more liquid when investor sentiment is higher still persists even after controlling for the effect of market...