Tolkien’s invented writing system used on the One Ring is Tengwar.
Tengwar, also called the Fëanorian script, is a writing system created in Tolkien’s fictional history by Fëanor. It appears in many forms across Middle-earth and can represent different languages, including Quenya, Sindarin, and the Black Speech. The Ring inscription is one of its most famous examples.
Tengwar should not be confused with the language of the inscription. The words are in the Black Speech of Mordor, while the letters are Tengwar characters. This is similar to the distinction between a spoken language and the alphabet or script used to write it.
Tolkien also created Cirth, a runic writing system associated especially with the Dwarves. Sarati was an earlier script in his mythology, and Angerthas refers to expanded runic systems. These neighboring terms make the Ring’s script a popular source of mix-ups.