Share repurchase, also known as share buyback or stock buyback, is the reacquisition by a company of its own shares. It represents an alternative way of returning money to shareholders instead of dividends. Repurchases allow stockholders to legally reduce taxes, where instead of paying tax on dividends they pay the lower taxes on the capital gains when selling the stock, whose price is now proportionally higher because of the smaller number of shares outstanding.

In most countries, a corporation can repurchase its own stock by distributing cash to existing shareholders in exchange for a fraction of the company's outstanding equity; that is, cash is exchanged for a reduction in the number of shares outstanding. The company either retires the repurchased shares or keeps them as treasury stock, available for reissuance.

Under U.S. corporate law, there are six primary methods of stock repurchase: open market, private negotiations, repurchase "put" rights, two variants of self-tender repurchase (a fixed price tender offer and a Dutch auction), and accelerate repurchases. More than 95% of the buyback programs worldwide are through an open-market method, whereby the company announces the buyback program and then repurchases shares in the open market (stock exchange). In the late 20th and the early 21st century, there was a sharp rise in the volume of share repurchases in the United States. Large share repurchases started later in Europe than in the United States, but are nowadays a common practice around the world.

U.S. Securities and Exchange Commission (SEC) rule 10b-18 sets requirements for stock repurchase in the United States. Rule 10b-18 provides a voluntary "safe harbor" from liability for market manipulation under Sections 9(a)(2) and 10(b) of the Securities Exchange Act of 1934.

Companies typically have two uses for profits. Firstly, some part of profits can be distributed to shareholders in the form of dividends or stock repurchases. The remainder of profits are retained earnings, kept inside the company and used for investing in the future of the company, if profitable ventures for reinvestment of retained earnings can be identified. However, sometimes companies may find that some or all of their retained earnings cannot be reinvested to produce acceptable returns.

Share repurchases are an alternative to dividends. When a company repurchases its own shares, it reduces the number of shares held by the public. The reduction of the float, or publicly traded shares, means that even if profits remain the same, the earnings per share increase.

Repurchases allow stockholders to delay taxes which they would have been required to pay on dividends in the year the dividends are paid, to instead pay taxes on the capital gains they receive when they sell the stock, whose price is now higher because of the smaller number of shares outstanding.

Aside from paying out free cash flow, repurchases may also be used to signal and/or take advantage of undervaluation. If a firm's manager believes their firm's stock is currently trading below its intrinsic value, they may consider repurchases. An open market repurchase, whereby no premium is paid on top of current market price, offers a potentially profitable investment for the manager. That is, they may repurchase the currently undervalued shares, wait for the market to correct the undervaluation whereby prices increase to the intrinsic value of the equity, and re-issue them at a profit. Alternatively, they may undertake a fixed price tender offer, whereby a premium is often offered over current market price; this sends a strong signal to the market that they believe that the firm's equity is undervalued, which is proven by their willingness to pay above market price to repurchase the shares. However, scholars also suggest that repurchases sometimes might be a cheap talk and convey a misleading signal due to the flexibility of repurchases.