A stock market bubble is a type of economic bubble taking place in stock markets when market participants drive stock prices above their value in relation to some system of stock valuation.

An economic bubble is a period when current asset prices greatly exceed their intrinsic valuation, being the valuation that the underlying long-term fundamentals justify. Bubbles can be caused by overly optimistic projections about the scale and sustainability of growth, and/or by the belief that intrinsic valuation is no longer relevant when making an investment. They have appeared in most asset classes, including equities, commodities, real estate, and even esoteric assets. Bubbles usually form as a result of either excess liquidity in markets, and/or changed investor psychology. Large multi-asset bubbles, are attributed to central banking liquidity.