The IBM 303X is a discontinued line of mainframe computers, the first model of which, the IBM 3033 Processor, nicknamed "The Big One", was introduced March 25, 1977.

Two additional processors, the 3031 and the 3032, were announced on October 6, 1977.

All three 303X systems were withdrawn on February 5, 1985.

The CPUs feature instruction pipelining, "several instructions can be pre-fetched while one is being executed". "Processor storage ... is four-way interleaved" resulting in "a significantly faster data rate than... non-interleaved". All three systems include a Dual-display console, the newly announced IBM 3036. The 3036 has an L-shaped desk, with two workstations. Each workstation has a service processor, a 3277 display, and two diskette drives (33FD). One diskette is used for logging hardware errors, and the other contains microcode. One of the service processors is the master, and the other is a backup; they can automatically switch roles when needed. The service processor is internally very similar to the service processor used in the IBM System/370 Model 158.

The systems consume less than half the floor space of a System/370 with an equal amount of computer memory and an identical number of channels because "the channels are physically integrated within the processor mainframe." All three machines have one or several dedicated I/O processors, called Channel Directors. A Channel Director is internally very similar to a IBM System/370 Model 158 CPU, running special channel microcode. Each Director has six channels attached, usually one byte multiplexor and five block multiplexor channels. The 3031 is air-cooled, while the two other processors are water-cooled. All three machine types use a 400 Hz power source. For the 3031 this is provided by a motor generator 3017, which is part of the processor complex; for the 3032 and 3033 an external 400 Hz power source is needed.

The initial announcement of the 3033 also introduced new operating system versions "MVS/System Extensions (MVS/SE) and VM/System Extensions (VM/SE)."

Beginning in 1977, IBM began to introduce new systems, using the following descriptions:

At this time, other companies, known as plug compatible manufacturers (PCMs), were competing with IBM by producing IBM-compatible systems. By the late 1970s and early 1980s, patented technology allowed Amdahl IBM-compatible mainframes of this era to be completely air-cooled, unlike IBM systems that required chilled water and its supporting infrastructure— the 8 largest of the 18 models of the ES/9000 systems introduced in 1990 were water-cooled; the other ten were air-cooled.