An activist shareholder is a shareholder who uses an equity stake in a corporation to put pressure on its management. A fairly small stake (less than 10% of outstanding shares) may be enough to launch a successful campaign. In comparison, a full takeover bid is a much more costly and difficult undertaking. The goals of activist shareholders range from financial (increase of shareholder value through changes in corporate policy, cost cutting, etc.) to non-financial (disinvestment from particular countries, etc.). Shareholder activists can address self-dealing by corporate insiders, although large stockholders can also engage in self-dealing to themselves at the expense of smaller minority shareholders.^: 241

According to research firm Insightia, a total of 967 listed companies globally were publicly subjected to activist demands in 2022, up from 913 in 2021. Shareholder activism can take any of several forms: proxy battles, publicity campaigns, shareholder resolutions, litigation, and negotiations with management. Daniel Loeb, head of Third Point Management, is notable for his use of sharply written letters directed towards the CEOs of his target companies.

Activism may help to address the principal-agent problem where the management (agents) do not adequately respond to the wishes of the principals (investors) of publicly traded companies. In the 2010s, investments in the activist asset class grew, with activists receiving coverage by the media and positive attention from investors. Activists have typically engaged in adversarial campaigns, but have also in some cases been able to acquire board seats with a formal proxy context.

Shareholder activists are making their mark on mergers and acquisitions as well – a 2015 survey of corporate development leaders found that 60% of respondents saw shareholder activism affecting transaction activity in their industry. Increasingly, however, the non-financial form of shareholder activism is affecting companies in a range of sectors. Shareholders, often with a comparatively small stake in a company, are seeking to influence the company's environmental and social performance.

Some of the recent activist investment funds include: California Public Employees' Retirement System (CalPERS), Icahn Management LP, Santa Monica Partners Opportunity Fund LP, State Board of Administration of Florida (SBA), and Relational Investors, LLC.

Due to the Internet, smaller shareholders have also gained an outlet to voice their opinions. In 2005, small MCI Inc. shareholders created an online petition to protest the MCI/Verizon merger.