Acorn System BASIC and Atom BASIC are two closely related dialects of the BASIC programming language developed by Acorn Computers for their early microcomputers like the Acorn System 3 and Acorn Atom. Developed in-house, they have a number of significant idiosyncrasies compared to most BASIC dialects of the home computer era.

In particular, the language lacked statements for many of the machine's internal functions and provided this using direct access and manipulation of memory locations using indirection operators instead of PEEK and POKE. Both also lacked floating-point support, although this could be added with an optional ROM which introduced further idiosyncrasies. System and Atom BASIC differ primarily in that Atom used the same indirection system to provide rudimentary string manipulation, which Standard lacked, and added a small number of new statements for computer graphics.

Most of these oddities were removed when the underlying system was greatly expanded to produce BBC BASIC on the Atom's successor, the BBC Micro. BBC BASIC ROMs were later offered to Atom users.

Acorn Computers formed in 1978 and got its start making a series of kit-built and Eurocard-based systems starting with the Acorn System 1 in 1979. They developed Acorn System BASIC for these machines, an integer-only dialect that required only 4 KB of memory in total. The language had a number of implementation details that made it "highly non-standard."

The Atom, introduced in 1980, was built from parts of the System 3 packaged onto a single board. Systems shipped standard with 2 KB of RAM and 8 KB of ROM, which included BASIC and a number of device drivers. Atom BASIC had only a few changes from the System version, adding support for string manipulation and a small number of graphics commands. The Atom was upgradable, with up to 12 KB of RAM in total and an additional 4 KB of ROM that added floating-point support. This used separate functions and operations that worked on them, indicated by the % symbol. This choice of symbol was unfortunate, as Microsoft BASIC used the percent sign to indicate integers, not floating point.

The Atom was on the market for only a short period before Acorn began development of its successor, the Proton. This was initially to be a two-processor unit. The design was still in its earliest stages when a series of events led to it being selected as the basis of the single-CPU BBC Micro. At the time, there were comments that it should definitely not use Acorn's variety of BASIC, which "virtually no other microcomputer can understand" and that "If the new language were based on the Atom's form of BASIC, it would be a disaster."

Ultimately, the BBC system did use those older Acorn-written BASIC variants, but heavily modified. The resulting BBC BASIC was much more similar to Microsoft BASIC and was later offered as an upgrade to the Atom.

As the two dialects are very similar, the following will refer to Atom BASIC primarily and point out differences where they exist.