MSN Messenger (also known colloquially simply as MSN), later rebranded as Windows Live Messenger, was a cross-platform instant-messaging client developed by Microsoft. It connected to the now-discontinued Microsoft Messenger service and, in later versions, was compatible with Yahoo! Messenger and Facebook Messenger. The service was discontinued in 2013 in favor of Skype, which was later replaced with Microsoft Teams.

The client was first released as MSN Messenger Service on July 22, 1999, and was marketed under the MSN brand until 2005, when it was rebranded under the Windows Live name. It has since been officially known by the latter name, although its first name remained in common use. In June 2009, Microsoft reported the service attracted over 330 million active users each month, placing it among the most widely used instant-messaging clients in the world.

Following its acquisition of Skype Technologies in May 2011, Microsoft added interoperability between Skype and Microsoft accounts, allowing Skype—which had features unique to its platform and a wider user base—to communicate with Windows Live Messenger contacts. In 2013, under the leadership of CEO Steve Ballmer, the product was discontinued, and Microsoft began cutting service to existing clients. It remained active in China for another 18 months and ceased operations there on October 31, 2014.

In 2018, a free version of Microsoft Teams became available after years as a component of an Office 365 subscription. Teams inherited the Microsoft accounts architecture, allowing users to invite contacts from the discontinued services, MSN Messenger and Skype.

Before the product was renamed Windows Live Messenger, it was named "MSN Messenger" from 1999 to 2006. During that time, Microsoft released seven major versions as follows. The first version of MSN Messenger Service, version 1.0 (1.0.0863), was released July 22, 1999. It included only basic features, such as plain text messaging and a simplistic contact list. When it was first released, it featured support for access to America Online's AIM network. America Online continually tried to block Microsoft from having access to their service until eventually the feature was removed, and it has not re-surfaced in any later versions of the software. AOL did this by exploiting a buffer overflow bug in AIM, which causes it to execute a bit of machine code sent by the server. When this code runs, it determines if the client is AIM and sends a message back to verify the client. Since then, the software has only allowed connections to its own service, requiring a Windows Live ID (.NET Passport at that time) account to connect. Microsoft released the first major update, version 2.0 (2.0.0083), on November 16, 1999. It included a rotating advertising banner and the ability to customize the appearance of the chat window. It came as an install option for Windows Me. This version was followed the next year by version 3.0 (3.0.0080), which was released May 29, 2000. It included file transfers and PC-to-PC and PC-to-phone audio capabilities with Net2Phone and Callserve, two of the larger VoIP providers.

Along with the release of Windows XP came version 4.6 of MSN Messenger, on October 23, 2001. It included major changes to the user interface, the ability to group contacts, and support for voice conversations. In this version, the client software was renamed from "MSN Messenger Service" to just "MSN Messenger", while the underlying service became known as ".NetWork Messenger Service". This version was only compatible with Windows 95, 98, ME, NT 4.0, and 2000, because Microsoft provided a scaled-down new program for Windows XP, called Windows Messenger. Version 5.0 of MSN Messenger was released on October 24, 2002. It was the first version that was allowed to be installed along with Windows Messenger on Windows XP. It included UPnP (Universal Plug and Play) based file transfers, minor changes to the user interface artwork, and a Windows Media Player interface plug-in.

Version 6.0 of MSN Messenger was released July 17, 2003. MSN Messenger 6.0 was a major overhaul of the whole platform, upgrading its simple text-based interface to include customizable elements such as emoticons, personalized avatars, and backgrounds. An update, version 6.1, focused on improvements to the conversation window, enabling users to hide the window frame and menu bar, and also the ability to change the theme color. The theme color could be set differently for each user. Another update, version 6.2, was released April 22, 2004, and it was the last version of the MSN Messenger 6 series. The most notable changes were a dedicated Mobile group for mobile contacts, a connection troubleshooter, and the Launch Site feature was renamed to Fun & Games.^{[citation needed]}

MSN Messenger received a major upgrade to version 7.0 on April 7, 2005. This version brought wink features that were previously only available in threedegrees. This version also advertised items to sell to you including animated display pictures, emoticons and backgrounds. The contact list window style was also updated to match instant message windows. This version also introduced the Xbox Live Integration feature. This version also introduced digital ink and handwriting recognition support. It is the last version of MSN Messenger to support Windows 98, Windows Me and Windows 2000.