Investments in sustainable assets have grown at a rapid pace over the last decade. Theory partially attributes this increase to changes in investor taste for sustainability. We develop a novel and direct measure of market-wide aggregate investor sentiment for sustainability and present evidence that it has a significant impact on asset prices. Using microblogging data from a social media investing platform, we construct a daily index capturing investor taste for environmental-, social-, and governance issues. We find that (i) sustainability sentiment temporarily increases during market downturns; (ii) positive sentiment shocks drive prices of sustainable stocks above their fundamental value, which is corrected in the long run, whereas negative shocks predict long-term asset price decreases for sustainable stocks; and (iii) sentiment forecasts flows into sustainable mutual funds.