The Tamil script, like the other Brahmic scripts, is thought to have evolved from the original Brahmi script. The earliest inscriptions which are accepted examples of Tamil writing date to a time just after the Ashokan period. The script used by these inscriptions is commonly known as the Tamil-Brahmi or “Tamili script”, and differs in many ways from standard Ashokan Brahmi. For example, early Tamil-Brahmi, unlike Ashokan Brahmi, had a system to distinguish between pure consonants (m in this example) and consonants with an inherent vowel (ma in this example). In addition, according to Iravatham Mahadevan, early Tamil Brahmi used slightly different vowel markers, had extra characters to represent letters not found in Sanskrit, and omitted letters for sounds not present in Tamil, such as voiced consonants and aspirates. Inscriptions from the second century use a later form of Tamil-Brahmi, which is substantially similar to the writing system described in the Tolkāppiyam, an ancient Tamil grammar. Most notably, they use the puḷḷi to suppress the inherent vowel. The Tamil letters thereafter evolved towards a more rounded form, and by the fifth or sixth century had reached a form called the early vaṭṭeḻuttu.

 Tampiran Vanakkam (Doctrina Christum) was the first book in Tamil, printed on 20 October 1578

The modern Tamil script does not, however, descend from this script. In the seventh century, the Pallava dynasty created a new script for Tamil, which was formed by simplifying the Grantha alphabet (which in turn derived from Southern Brahmi), and adding to it the Vaṭṭeḻuttu alphabet for sounds not found in Sanskrit. By the 8th century, this new script supplanted Vaṭṭeḻuttu in the Chola and Pallava kingdoms which lay in the north portion of the Tamil-speaking region. Vaṭṭeḻuttu continued to be used in the southern portion of the Tamil-speaking region, in the Chera and Pandyan kingdoms until the 11th century, when the Pandyan kingdom was conquered by the Cholas.

A Tamil book printed in 1781

Over the next few centuries, the Chola-Pallava script evolved into the modern Tamil script. The use of palm leaves as the primary medium for writing led to changes in the script. The scribe had to be careful not to pierce the leaves with the stylus while writing, because a leaf with a hole was more likely to tear and decay faster. As a result, the use of the puḷḷi to distinguish pure consonants became rare, with pure consonants usually being written as if the inherent vowel were present. Similarly, the vowel marker for the kuṟṟiyal ukaram, a half-rounded u which occurs at the end of some words and in the medial position in certain compound words, also fell out of use and was replaced by the marker for the simple u. The puḷḷi did not fully reappear until the introduction of printing, but the marker kuṟṟiyal ukaram never came back into use, although the sound itself still exists and plays an important role in Tamil prosody.

The forms of some of the letters were simplified in the nineteenth century to make the script easier to typeset. In the twentieth century, the script was simplified even further in a series of reforms, which regularised the vowel markers used with consonants by eliminating special markers and most irregular forms.