A stock market bubble is a type of economic bubble taking place in stock markets when price of stocks rise and become overvalued by any measure of stock valuation.

The existence of stock market bubbles is at odds with the assumptions of the efficient-market hypothesis which assumes rational investor behaviour. Behavioral finance theory attribute stock market bubbles to cognitive biases that lead to groupthink and herd behavior. Bubbles occur not only in real-world markets, with their inherent uncertainty and noise, but also in highly predictable experimental markets. In the laboratory, uncertainty is eliminated and calculating the expected returns should be a simple mathematical exercise, because participants are endowed with assets that are defined to have a finite lifespan and a known probability distribution of dividends. Other theoretical explanations of stock market bubbles have suggested that they are rational, intrinsic, and contagious.