Tengwar is the script Tolkien used for much of the writing on the One Ring.
The Ring inscription is written in Tengwar characters, using the Black Speech of Mordor. The famous text begins with “Ash nazg durbatulûk,” meaning “One Ring to rule them all,” and continues with the verse describing the other Rings and Sauron’s power.
Tolkien designed Tengwar as a flexible writing system for the languages of Middle-earth. Its letters, or tehtar and related signs, can represent consonants and vowels in different modes. The script is especially associated with Quenya and Sindarin, though the Ring inscription uses it for the Black Speech.
Cirth is a different Tolkien script, characterized by angular runes and used especially by Dwarves. The distinction matters because both scripts appear across Tolkien’s legendarium, but the Ring’s inscription is Tengwar.