The Macintosh Quadra 900 is a high-end personal computer introduced with the Quadra 700 in October 1991 as Apple Computer's first computers in the Quadra series using the Motorola 68040 processor. It was discontinued in 1992, and succeeded by the very similar Quadra 950. The Quadra 900 was more expandable than the Quadra 700 but cost US$7,200 (over US$12,000 in 2015 dollars). The Quadra 900 could be upgraded to 256 megabytes of RAM—an astronomical amount for the time, when a typical midrange system would come equipped with 2–4 MB. The standard, as-shipped configuration for the 900 was 4 MB. The high RAM and storage capacity, along with the expandability of five NuBus 90 slots and fast 25 MHz processor, made it a very useful computer for scientific or design work. It was the first Mac to be built in a full tower case, featuring a single 5.25-inch drive bay initially intended to hold a tape backup drive but often repurposed to hold a CD-ROM or SyQuest drive during the machines' operational lifespan. Quadra 900s and 950s with full complements of RAM were exceedingly rare with 16–30-pin SIMM RAM slots, due to the high cost (many thousands of dollars) of the SIMMs at the time. It is the earliest Macintosh model able to run Mac OS 8.
With the Power Macintosh Upgrade Card installed, the Quadra 900 runs at 50 MHz and its name is reported in the System Profiler as the Power Macintosh 900.