Rhapsody is an operating system that was developed by Apple Computer after its purchase of NeXT in the late 1990s. It is the fifth major release of the Mach-based operating system that was developed at NeXT in the late 1980s, previously called OpenStep and NeXTSTEP. Rhapsody was targeted to developers for a transition period between the Classic Mac OS and Mac OS X. Rhapsody represented a new and exploratory strategy for Apple, more than an operating system, and runs on x86-based PCs and on Power Macintosh.

Rhapsody's OPENSTEP-based Yellow Box API frameworks were ported to Windows NT for creating cross-platform applications. Eventually, the non-Apple platforms were discontinued, and later versions consist primarily of the OpenStep operating system ported to Power Macintosh, merging the Copland-originated GUI of Mac OS 8 with that of OPENSTEP. Several existing classic Mac OS frameworks were ported, including QuickTime and AppleSearch. Rhapsody can run Mac OS 8 and its applications in a paravirtualization layer called Blue Box for backward compatibility during migration to Mac OS X.

Rhapsody follows Apple's pattern through the 1990s of music-related codenames for operating system releases (see Rhapsody (music)). Apple had canceled its previous next-generation operating system strategy of Copland (named for American composer, Aaron Copland) and its pre-announced successor Gershwin (named for George Gershwin, composer of Rhapsody in Blue). Other musical code names include Harmony (Mac OS 7.6), Tempo (Mac OS 8), Allegro (Mac OS 8.5), and Sonata (Mac OS 9).

In the mid-1990s, Mac OS was falling behind Windows. In 1993, Microsoft had introduced the next-generation Windows NT, which was a processor-independent, multiprocessing and multi-user operating system. At the time, Mac OS was still a single-user OS, and had gained a reputation for being unstable. Apple made several attempts to develop modern replacements for Mac OS, which all failed, harming public confidence in the company, while Macintosh sales continued to decline. Apple's most promising next-generation operating system, Copland, was mismanaged and had to be abandoned in 1996. In response, Apple CEO Gil Amelio decided to acquire or license an already-built operating system from another company. Apple's executive team considered BeOS, NeXT's NeXTSTEP, Sun Microsystems' Solaris, and Windows NT, and eventually acquired NeXT in December 1996.

Rhapsody was announced by Gil Amelio at the January 7, 1997 Macworld Expo SF, and first demonstrated at the May 1997 Worldwide Developers Conference (WWDC).

At Macworld SF 1997, Steve Jobs said that Mac OS and its Macintosh Toolbox placed developers on the "fifth floor", Windows NT on the "seventh floor", while Rhapsody's OpenStep libraries would let developers start on the "twentieth floor". Jobs argued that this would make it faster to implement existing apps, and would allow even small developer teams to create innovative apps. At the time, fewer than 25% of Americans used computers, and many thought the computing industry was "just waiting to take off"; most existing applications were office tools, and "the software for the vast majority of people [had] yet to be written". Jobs gave the examples of Lighthouse Design, which had created an entire office suite for NeXTSTEP with only 18 employees; Tim Berners-Lee, who had created WorldWideWeb, the first web browser, on NeXTSTEP; and Dell, which created its first eCommerce website using NeXT's WebObjects.

Since Yellow Box was also available on Windows as a dynamic-link library (DLL), Yellow Box apps would be able to run on both Rhapsody and Windows through a recompile. Microsoft provided no equivalent that would enable Win32 apps to run on other platforms. Additionally, Rhapsody itself could be installed on either PowerPC Macintoshes or Intel x86 PCs, enlarging developers' potential customer base.

Apple pitched Rhapsody to users by emphasizing its improved stability and responsiveness. For example, on classic Mac OS, incoming faxes could leave the machine frozen until the fax completes; and if an app crashed, it could take down the whole system and require a reboot, potentially leading to data loss. Additionally, Rhapsody would inherit NeXTSTEP's local network sharing functionality, and make it easy to share data across apps. At Macworld SF 1997, Steve Jobs vaunted NeXTSTEP's maturity by pointing out that it had been adopted by many large organizations, including Wall Street banks and the CIA. BYTE magazine described NeXTSTEP as "field-tested, time-proven code" in January 1998, ten years after the operating system's introduction.