Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated as IE or MSIE) is a retired 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 and ended support on June 15, 2022 for Windows 10 Semi-Annual Channel (SAC), 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. It has since fallen out of general use after retirement. 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. 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 determined that the integration of Internet Explorer with Windows has been to the detriment of fair browser competition.

Internet Explorer 7 was supported on Windows Embedded Compact 2013 until October 10, 2023. The core of Internet Explorer 11 will continue being shipped and supported until at least 2029 as IE Mode, a feature of Microsoft Edge, enabling Edge to display web pages using Internet Explorer 11's Trident layout engine and other components. Through IE Mode, the underlying technology of Internet Explorer 11 partially exists on versions of Windows that do not support IE11 as a proper application, including newer versions of Windows 10, as well as Windows 11, Windows Server Insider Build 22463 and Windows Server Insider Build 25110.