The writing system J.R.R. Tolkien created for the Ring inscription and many Elvish texts is Tengwar. The name comes from Quenya and means “letters” or “signs.”
Tengwar is an alphabetic script whose characters represent consonants and whose vowel marks, called tehtar, can be placed around them. Tolkien designed it as part of his broader linguistic world-building, giving it historical relationships and different usage traditions within Middle-earth.
The inscription on the One Ring is written in Tengwar, although the language of the inscription is the Black Speech of Mordor rather than an Elvish language. This distinction is an important source of confusion: a script and a language are not the same thing.
Cirth is another Tolkien writing system, associated especially with runic forms used by Dwarves and other peoples. Sarati is an earlier script in Tolkien’s invented writing history, while Angerthas refers to expanded runic arrangements. Tengwar is the specific script seen on the Ring.