The script on the One Ring in J.R.R. Tolkien’s The Lord of the Rings is Tengwar.
Tengwar is a writing system created by Tolkien for the languages of his legendarium. The Ring inscription is written with Tengwar letters, but the words themselves are in the Black Speech of Mordor. This distinction matters: a script is a system of written symbols, while a language is the spoken or structured linguistic system represented by those symbols.
The inscription begins with the famous words “Ash nazg durbatulûk,” describing one Ring’s power over the others. In Tolkien’s fictional history, the inscription was made by Sauron in the fires of Mount Doom.
Tolkien also created other scripts, including Cirth, which uses angular runes associated especially with Dwarves and other peoples.