A market maker or liquidity provider is a company or individual that quotes both a buy and a sell price in a tradable asset held in inventory, hoping to make a profit on the difference, which is called the bid–ask spread or turn. This stabilizes the market, reducing price variation (volatility) by setting a trading price range for the asset.

In U.S. markets, the U.S. Securities and Exchange Commission defines a "market maker" as a firm that stands ready to buy and sell stock on a regular and continuous basis at a publicly quoted price. A Designated Primary Market Maker (DPM) is a specialized market maker approved by an exchange to guarantee a buy or sell position in a particular assigned security, option, or option index.

Most foreign exchange trading firms are market makers, as are many banks. The foreign exchange market maker both buys foreign currency from clients and sells it to other clients. They derive income from the trading price differentials, helping the market by providing liquidity, reducing transaction costs, and facilitating trade.

Market makers that stand ready to buy and sell stocks listed on an exchange, such as the New York Stock Exchange (NYSE) or the London Stock Exchange (LSE), are called "third market makers". Most stock exchanges operate on a "matched bargain" or "order driven" basis. When a buyer's bid price meets a seller's offer price or vice versa, the stock exchange's matching system decides that a deal has been executed. In such a system, there may be no designated or official market makers, but market makers nevertheless exist.

As of October 2008,^([update]) there were over two thousand market makers in the United States, and over one hundred in Canada.

In the United States, the NYSE and NYSE American, among others, have designated market makers, formerly known as "specialists", who act as the official market maker for a given security. The market makers provide a required amount of liquidity to the security's market, and take the other side of trades when there are short-term buy-and-sell-side imbalances in customer orders. In return, the specialist is granted various informational and trade execution advantages.

Other U.S. exchanges, most prominently the NASDAQ stock exchange, employ several competing official market makers in a security. These market makers are required to maintain two-sided markets during exchange hours and are obligated to buy and sell at their displayed bids and offers. They typically do not receive the trading advantages a specialist does, but they do get some, such as the ability to naked short a stock, i.e., selling it without borrowing it. In most situations, only official market makers are permitted to engage in naked shorting.

Changes to the rules in the 2000s and 2010s have explicitly banned naked shorting by options market makers.

In liquid markets like the NYSE, nearly every asset has open interest, providing two benefits: price takers can buy or sell at any time, and observers can continually monitor a precise price of every asset.

On the LSE, there are official market makers for many securities. Some of the LSE's member firms take on the obligation of always making a two-way price in each of the stocks in which they make markets. Their prices are the ones displayed on the Stock Exchange Automated Quotation (SEAQ) system and it is they who generally deal with brokers buying or selling stock on behalf of clients.

Proponents of the official market making system claim market makers add to the liquidity and depth of the market by taking a short or long position for a time, thus assuming some risk in return for the chance of a small profit. On the LSE, one can always buy and sell stock: each stock always has at least two market makers and they are obliged to deal.

In contrast, on smaller, order-driven markets such as the JSE Securities Exchange it can be difficult to determine the buying and selling prices of even a small block of stocks that lack a clear and immediate market value because there are often no buyers or sellers on the order board.