Pump and dump (P&D) is a form of securities fraud that involves artificially inflating the price of an owned stock through false and misleading positive statements (pump), in order to sell the cheaply purchased stock at a higher price (dump).

In a classic pump and dump, the scammer first buys lots of cheap shares in some company of little value. They then spread false information exaggerating the potential value of the company. This causes investors and speculators to buy shares in said company from other parties, causing the price of the shares to rise. When the share price has reached a level that pleases the scammer, they sell all their shares. This causes the share price to drop.

When the investors and speculators realize that they overestimated the potential of these shares, they in turn sell their shares in an attempt to cut their losses, and the price drops even faster. Because the investors sold their shares for less than what they paid for them, they lose money.

While fraudsters in the past relied on cold calls, the Internet now offers a cheaper and easier way of reaching large numbers of potential investors through spam email, investment research websites, social media, and general misinformation campaigns.

Pump-and-dump schemes occur on the Internet using an email spam campaign, through media channels via a fake press release, or through telemarketing from "boiler room" brokerage houses (such as that dramatized in the 2000 film Boiler Room). Often the stock promoter will claim to have "inside" information about impending news. Newsletters would purport to offer unbiased recommendations, then tout a company as a "hot" stock, for their own benefit. Promoters also post messages in online chat groups or internet forums, urging readers to buy the stock quickly.

If a promoter's campaign to "pump" a stock is successful, it will entice unwitting investors to purchase shares of the target company. The increased demand, price, and trading volume of the stock could convince more people to believe the hype and to buy shares as well. When the promoters behind the scheme sell (dump) their shares and stop promoting the stock, the price plummets, and other investors are left holding a stock that is worth significantly less than what they paid for it.

Fraudsters frequently use this ploy with small, thinly traded companies—known as "penny stocks", generally traded over-the-counter (in the United States, this would mean markets such as the OTC Bulletin Board or the Pink Sheets), rather than markets such as the New York Stock Exchange (NYSE) or NASDAQ—because it is easier to manipulate a stock when there is little or no independent information available about the company. The same principle applies in the United Kingdom, where target companies are typically small companies on the AIM or OFEX.

In the early 1990s, the penny-stock brokerage Stratton Oakmont artificially inflated the price of owned stock through false and misleading positive statements in order to sell the cheaply purchased stock at a higher price. Firm co-founder Jordan Belfort was criminally convicted for applying the scheme. He later turned his story into a memoir, The Wolf of Wall Street, which was later adapted into an Academy Award–nominated film of the same name.

During the dot-com bubble, when US stock-market fever was at its height and many people spent significant amounts of time on stock Internet message boards, a 15-year-old named Jonathan Lebed allegedly used the Internet to run a successful pump and dump. Lebed bought penny stocks and then promoted them on message boards, pointing at the price increase. Allegedly, when other investors bought the stock, Lebed sold his for a profit, leaving the other investors holding the bag. He came to the attention of the U.S.

Securities and Exchange Commission (SEC), which filed a civil suit against him alleging security manipulation. Lebed settled the charges by paying a fraction of his total gains. He neither admitted nor denied wrongdoing, but promised not to manipulate securities in the future.

As late as April 2001, before Enron collapsed, executives at the company participated in an elaborate pump and dump scheme. Studies of the anonymous messages posted on the Yahoo board dedicated to Enron revealed predictive messages that it was akin to a house of cards, had "fooled even the most sophisticated analysts", and that investors should bail out while the stock was good. After Enron falsely reported profits, which inflated the stock price, they covered the real numbers by using questionable accounting practices.

Twenty-nine Enron executives sold overvalued stock for more than a billion dollars before the company went bankrupt.