Siddhāṃ (also known as Kutila) is an Indic script used in India from the 6th century to the 13th century. Also known in its later evolved form as Siddhamātṛkā, Siddham is a medieval Brahmic abugida, derived from the Gupta script and ancestral to the Nāgarī, Eastern Nagari, Tirhuta, Odia and Nepalese scripts. The Siddham script was widely used by Indian Buddhists and still remains in use by East Asian Buddhists, especially for writing mantras, seed syllables, and dharanis.

The word Siddhaṃ means "accomplished", "completed" or "perfected" in Sanskrit. The script received its name from the practice of writing Siddhaṃ, or Siddhaṃ astu ('may there be perfection'), at the head of documents. Other names for the script include bonji (Japanese: 梵字) "Brahma's characters" and "Sanskrit script" and Chinese: 悉曇文字; pinyin: Xītán wénzi "Siddhaṃ script".

The Siddham script evolved from the Gupta Brahmi script in the late 6th century CE.

Many Buddhist texts taken to China along the Silk Road were written using a version of the Siddhaṃ script. This continued to evolve, and minor variations are seen across time, and in different regions. Importantly, it was used for transmitting the Buddhist tantra texts. At the time it was considered important to preserve the pronunciation of mantras, and Chinese was not suitable for writing the sounds of Sanskrit. This led to the retention of the Siddhaṃ script in East Asia. The practice of writing using Siddhaṃ survived in East Asia where Chinese Esoteric Buddhism persisted.

Kūkai introduced the Siddhaṃ script to Japan when he returned from China in 806, where he studied Sanskrit with Nalanda-trained monks including one known as Prajñā (Chinese: 般若三藏; pinyin: Bōrě Sāncáng; 734–c. 810). By the time Kūkai learned this script, the trading and pilgrimage routes over land to India had been closed by the expanding Abbasid Caliphate.

In the middle of the 9th century, China experienced a series of purges of "foreign religions", thus cutting Japan off from the sources of Siddhaṃ texts. In time, other scripts, particularly Devanagari, replaced Siddhaṃ in India, while Siddhaṃ's northeastern derivative called Gaudi evolved to become the Eastern Nagari, Tirhuta, Odia and also the Nepalese scripts in the eastern and northeastern regions of South Asia, leaving East Asia as the only region where Siddhaṃ is still used.

There were special forms of Siddhaṃ used in Korea that varied significantly from those used in China and Japan, and there is evidence that Siddhaṃ was written in Central Asia, as well, by the early 7th century.

As was done with Chinese characters, Japanese Buddhist scholars sometimes created multiple characters with the same phonological value to add meaning to Siddhaṃ characters. This practice, in effect, represents a 'blend' of the Chinese style of writing and the Indian style of writing and allows Sanskrit texts in Siddhaṃ to be differentially interpreted as they are read, as was done with Chinese characters that the Japanese had adopted. This led to multiple variants of the same characters.