The Ancient South Arabian script (Old South Arabian: 𐩣𐩯𐩬𐩵, romanized: ms³nd; modern Arabic: الْمُسْنَد musnad) branched from the Proto-Sinaitic script in about the late 2nd millennium BCE, and remained in use through the late sixth century CE. It is an abjad, a writing system where only consonants are obligatorily written, a trait shared with its predecessor, Proto-Sinaitic, as well as some of its sibling writing systems, including Arabic and Hebrew. It is a predecessor of the Ge'ez script, and a sibling script of the Phoenician alphabet and, through that, the modern Latin, Cyrillic, and Greek alphabets.

The script is really two variants: the monumental and the miniscule script, the former for inscriptions, the latter scratched with wooden sticks. The scripts have a common origin but evolved into separate systems.

The earliest instances of the Ancient South Arabian (ASA) script are painted pottery sherds from Raybun in Hadhramaut in Yemen, which are dated to the late 2nd millennium BCE. It is an abjad script, meaning that only consonants are usually written in the script, with vowels inferred from context; it shares this feature both with its predecessor, the Proto-Sinaitic script, and modern Semitic languages. It is unclear precisely how and when the ASA script diverged from Proto-Sinaitic script, as inscriptions from its earliest days are rare. As with these other abjads, some vowels can be indicated if necessary, by including diacritical markings, called matres lectionis.^{[citation needed]}

Its mature form was reached around 800 BCE, and it remained use in more or less the same form until the 6th century CE. In those centuries, it was used to write multiple languages of the Southern Arabian peninsula and the Horn of Africa, including Sabaic, Qatabanic, Hadramautic, Minaean, Hasaitic, and Geʽez. It was eventually displaced by the modern Arabic alphabet during the early years of the spread of Islam. The modern Arabic writing system is related to the ASA script, as both are children of the Proto-Sinaitic script, but modern Arabic derives from the Phoenician and Nabatean scripts rather than ASA.

The Geʽez script is the sole extant writing system that derives from ASA. Unlike ASA, Geʽez is an abugida; the primary characters are pairs of consonants and vowels, with each character representing a syllable. Geʽez has been used to write Amharic, Tigrinya and Tigre, as well as other languages (including various Semitic, Cushitic, Omotic, and Nilo-Saharan languages). ASA is also a sibling of the Phoenician – the ancestor of most of the modern European alphabets, such as Latin, Cyrillic and Greek.

The Musnad script differs from the Arabic script, which most linguists believe developed from the Nabataean script in the fourth century AD, which in turn developed from the Aramaic script. The languages of the Southern Musnad script also differ greatly from the Northern Arabic language in terms of script, lexicon, grammar, styles, and perhaps sounds, and the letters of the script increase. The Musnad is derived from Arabic with an extra sibilant letter (some call it sāmikh) or the third sīn.

Six signs are used for numbers:

The sign for 50 was evidently created by removing the lower triangle from the sign for 100. The sign for 1 doubles as a word separator. The other four signs double as both letters and numbers. Each of these four signs is the first letter of the name of the corresponding numeral.