Sini (from Arabic: ٱلْخَطُ ٱلصِّينِيُّ, Al-khaṭ as-ṣīnī, lit. 'The Chinese script') is a calligraphic style used in China for the Arabic script. While Sini Script can refer to any type of Arabic Calligraphy influenced by Chinese Calligraphy, it exists on a spectrum in which the amount of Chinese influence increases as it is found further East. While Sini script resembles thuluth script, it is adapted to local styles in Chinese Mosques. Although Sini script exists on a broad spectrum, the most well-known form of Sini script, standardized during the Ming Dynasty, is characterized by its "round, flowing" Arabic letters featuring the "tapered" style more commonly found in Chinese calligraphy. It is also characterized by its thick horizontal and fine vertical strokes, a result that is due to the use of a brush rather than a qalam, which is the traditional writing pen for Islamic calligraphy.

One notable Sini calligrapher is Hajji Noor Deen Mi Guangjiang (b. 1963).

In Chinese mosques, Sini calligraphy is found on a variety of different surfaces including walls, boards and tablets made from stone or wood, and pillars made of concrete or stone, particularly near the mihrab or prayer niche. The wooden boards containing Sini calligraphy found at the entrance of the prayer hall often contain a "tasmiya" or invocation. Sini calligraphy may be inscribed both horizontally or vertically depending on the surface.

The vertical form of Sini Calligraphy, often found hanging on both sides of a doorway or on pillars, is influenced by the vertical style of Chinese calligraphy, particularly found in yinglian or duilian, the calligraphic artform of rhyming couplets. A particularly distinct form of vertical Sini calligraphy can be achieved by following the Chinese tradition which contains the calligraphy within a box or square form and tilting it on its axis to form diamond-shaped calligraphy. Examples of diamond-shaped Sini calligraphy can be found at the Great Mosque of Xi'an on columns flanking the prayer hall.

Sini script is also found on various objects produced for both local use and to be exported to the Muslim world. The David Collection in Copenhagen includes examples of bronze objects including a double-handed vessel with Chinese imagery dating to the seventeenth century and a bronze incense burner made during the reign of Ming emperor Zhengde, both inscribed with Sini calligraphy.

Among Islamic manuscripts in China, Sini script can be found in many Qur'ans produced in China throughout the Ming and Qing dynasties. Examples include a Qur'an from China dated to 1013/1605 in the Nasser D. Khalili Collection of Islamic Art (QUR992) and a Qur'an from China dated to the 16th-18th century in the Tareq Rajab Museum (TSR-MS-11). In these standardized 30-volume Qur'ans, most pages contain five to seven lines of script while the opening pages generally contain three lines. The script itself is generally large and wide-spaced. Although these Qur'ans may contain a variety of different scripts, the most frequently used script is derived from muhaqqaq.

Specific calligraphic features separating Chinese Qur'anic script from its prototype may be noted. Certain letters from the Arabic alphabet are extended. For example, the tails of letters ra, za, waw as well as the terminals nun and sad rendered below the baseline extend further horizontally. Additionally, the initial of ba, particularly on the bismillah, tends to extend vertically yet in a curved manner. The terminal mim is rendered unconventionally, with either its tail hanging vertically rather than in parallel structure with the "sublinear" tails of other letters, or having a "curvilinear" form. In general, Chinese Qur'anic script contains more curlicues and sharper letter angles.

More decorative Sini calligraphy can be seen on the roundels exhibited on the double pages which precede each juz or section of the Qur'an. Depending on the Qur'an, the calligraphy may be more or less "Sinified." A greater influence of Chinese stylistic elements on script may reduce its legibility to those unfamiliar to it.