The term Aljamiado denotes a modified Arabic (and, less frequently, Hebrew) script used for writing the Hispano-Romance languages spoken by communities of covert Muslims in Spain and Portugal known as Moriscos, from the 9th until the 16th centuries. Muslim speakers of Castilian and Aragonese in particular used the script. The term is also sometimes used as a general term for any spoken Hispano-Romance dialect of Mediterranean Muslims. Aljamiado writing first became widely used in the 15th century and continued until the 16th century, when King Philip II of Spain outlawed the used of Arabic, spoken or written, and gave the Moriscos three years to learn the language of the Christian Spanish. Aljamiado texts only came to the attention of the outside world in the early nineteenth century.

There are currently about 200 Aljamiado manuscripts in existence, mostly housed in museums and libraries around Madrid. They contain studies of death and judgement, travel literature, love stories, legends, moral teachings, poetry, books of proverbs and dreams, anti-christian and anti-jewish texts, legal documents, and a great number of Biblical and Quranic stories.

Aljamiado writing was never standardized, so different scribes represented the same phonemes differently, but some conventions were generally maintained by the majority of writers.

Some Arabic letters represent sounds which are not found in Aljamiado speech, for example gutterals. These letters were not used in Aljamiado texts, except for writing Arabic words. (Most extant texts include at least one Arabic passage or quotation.) Conversely, some Hispanic sounds could not be represented using Arabic letters, for example [ɲ] (ñ), [p] and the trilled [r]. These were represented by writing the letter which most closely approximated the desired sound, twice.

Arabic writing employs three vowel letters, representing the long vowels a (aleph), i (ya) and u (waw). The short counterparts of these are optionally written using diacritics. However, Aljamiado has five vowels; a, i, e, o, u. To represent the extra two vowels, Aljamiado uses the Arabic waw for both [o] and [u]. The reader must deduce which representation is intended from the context. [e] is written using the aleph symbol plus a fatha diacritic above the following letter.

Arabic syllables only ever consist of a consonant plus a (long or short) vowel, or a consonant plus a short vowel plus a closing consonant. The Arabic script is therefore designed to accommodate these syllable structures. However, spoken Aljamiado has a greater variety of permissible syllable structures to represent. Aljamiado syllables beginning with a consonant are written with a silent letter 'h' at the start. Syllables containing a consonant cluster must either be resyllabified to fit an Arabic syllable structure - so the word 'agradable' (nice) is written with the Arabic symbols for 'hag'+'ra'+'dab'+'le' - or an epenthetic vowel letter must be inserted between consonant letters - so the word 'clave' (key) is written with the symbols for 'ca'+'la'+'ve'. Syllables containing two vowels next to each other are written with an epenthetic consonant inserted between the two vowels. The consonant must be similar in quality to the first vowel in the pair. For example, 'ciencia' (science) is written with the Arabic symbols for ci+yen+ci+ya.

Aljamiado writing is unicameral like Arabic, that is, there is no upper and lower case. No full stops, commas or accents are used.