Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated as IE or MSIE) is a deprecated (or discontinued for most Windows editions) series of graphical web browsers developed by Microsoft that were used in the Windows line of operating systems. While IE has been discontinued on most Windows editions, it remains supported on certain editions of Windows, such as Windows 10 LTSB/LTSC. Starting in 1995, it was first released as part of the add-on package Plus! for Windows 95 that year. Later versions were available as free downloads or in-service packs and included in the original equipment manufacturer (OEM) service releases of Windows 95 and later versions of Windows. Microsoft spent over US$100 million per year on Internet Explorer in the late 1990s, with over 1,000 people involved in the project by 1999. New feature development for the browser was discontinued in 2016 in favor of its successor, Microsoft Edge.

Internet Explorer was once the most widely used web browser, attaining a peak of 95% usage share by 2003. This came after Microsoft used bundling to win the first browser war against Netscape, which was the dominant browser in the 1990s. Its usage share has since declined with the launches of Firefox (2004) and Google Chrome (2008) and with the growing popularity of mobile operating systems such as Android and iOS that do not support Internet Explorer. Estimates for Internet Explorer's market share in 2023 are about 0.25% across all platforms, or, by StatCounter's numbers, ranked 10th after 360 Safe Browser. On traditional PCs, the only platform on which it has ever had a significant share, it is ranked 7th at 0.62%, after 360 Safe Browser. Microsoft Edge, IE's successor, first overtook Internet Explorer in terms of market share in November 2019. Versions of Internet Explorer for other operating systems have also been produced, including an Xbox 360 version called Internet Explorer for Xbox and for platforms Microsoft no longer supports: Internet Explorer for Mac and Internet Explorer for UNIX (Solaris and HP-UX), and an embedded OEM version called Pocket Internet Explorer, later rebranded Internet Explorer Mobile, made for Windows CE, Windows Phone, and, previously, based on Internet Explorer 7, for Windows Phone 7.

The browser has been scrutinized throughout its development for its use of third-party technology (such as the source code of Spyglass Mosaic, used without royalty in early versions) and security and privacy vulnerabilities, and the United States and the European Union have alleged that the integration of Internet Explorer with Windows has been to the detriment of fair browser competition.