Theta (upper case Θ, lower case θ) is the eighth letter of the Greek alphabet. In the system of Greek numerals it has a value of 9.

The lower-case letter θ is used as a symbol for:

• A plane angle in geometry.
• θ is the voiceless (inter)dental fricative in the International Phonetic Alphabet.

The upper-case letter Θ is used as a symbol for:

• Earth Day

The Cyrillic letter Fita arose from Theta.