AOL Instant Messenger (AIM, sometimes stylized as aim) was an instant messaging and presence information computer program created by AOL that operated from 1997 to 2017. It used the proprietary OSCAR instant messaging protocol and the TOC protocol to allow users to communicate in real time.

AIM launched in May 1997 and became popular by the late 1990s; teens and college students were known to use the messenger's away message feature to keep in touch with friends, often frequently changing their away message throughout a day or leaving a message up with one's computer left on to inform buddies of their ongoings, location, parties, thoughts, or jokes.

AIM's popularity declined during the 2000s and 2010s as AOL subscribers started decreasing and as Gmail's Google Talk, SMS, and Internet social networks like Facebook gained popularity. Its fall has often been compared with other once-popular Internet services, such as Myspace. In June 2015, AOL was acquired by Verizon Communications. In June 2017, Verizon combined AOL and Yahoo into its subsidiary Oath Inc. (now called Yahoo). The company discontinued AIM as a service on December 15, 2017.

In May 1997, AIM was released unceremoniously as a stand-alone download for Microsoft Windows. AIM was an outgrowth of "online messages" in the original platform written in PL/1 on a Stratus computer by Dave Brown. At one time, the software had the largest share of the instant messaging market in North America, especially in the United States (with 52% of the total reported as of 2006^[update]). This does not include other instant messaging software related to or developed by AOL, such as ICQ and iChat.

During its heyday, its main competitors were ICQ (which AOL acquired in 1998), Yahoo! Messenger and MSN Messenger. AOL particularly had a rivalry or "chat war" with PowWow and Microsoft, starting in 1999. There were several attempts from Microsoft to simultaneously log into their own and AIM's protocol servers. AOL was unhappy about this and started blocking MSN Messenger from being able to access AIM. This led to efforts by many companies to challenge the AOL and Time Warner merger on the grounds of antitrust behaviour, leading to the formation of the OpenNet Coalition.

Official mobile versions of AIM appeared as early as 2001 on Palm OS through the AOL application. Third-party applications allowed it to be used in 2002 for the Sidekick. A version for Symbian OS was announced in 2003, as were others for BlackBerry and Windows Mobile.

After 2012, stand-alone official AIM client software included advertisements and was available for Microsoft Windows, Windows Mobile, Classic Mac OS, macOS, Android, iOS, and BlackBerry OS.

After seeing it's popularity peak between 1999 and 2005, AIM began to very slowly lose its daily active user base starting with the widespread adoption of SMS text messaging in the United States that had occurred over the same period followed by the quick rise of Gmail and its 2005 introduction of its built-in real-time chat feature Google Talk. By 2011, apps like Apple iMessage, social network messaging apps like Facebook Messenger, and mobile-first messaging apps such as WhatsApp had greatly reduced the user base of AIM and other desktop-centric competitors of its generation. AOL sought to compete by adding features such as integration with Google Talk and enabling inbound and outbound SMS text messaging between AIM and any mobile number.