Ichthys, from Koine Greek: ἰχθύς, (capitalized ΙΧΘΥΣ or ΙΧΘΥϹ) is the Greek word for "fish". (Alternative transliterations include Ichthus and Ikhthus). In English, this refers to a symbol consisting of two intersecting arcs, the ends of the right side extending beyond the meeting point so as to resemble the profile of a fish, used by early Christians as a secret Christian symbol and now known colloquially as the "sign of the fish" or the "Jesus fish."

ΙΧΘΥΣ (Ichthys) is an acronym for "Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ", (Iēsous Christos, Theou Yios, Sōtēr), which translates into English as "Jesus Christ, God's Son, Savior".

This explanation is given among others by Augustine in his Civitate Dei , where he notes that the generating sentence "Ίησοῦς Χειστός Θεοῦ Υἱός Σωτήρ" has 27 letters, i.e. 3 x 3 x 3, which in that age indicated power. Augustine quotes also an ancient text from the Sibylline oracles whose verses are an acrostic of the generating sentence.

Historians say the 20th-century use of the ichthys motif is an adaptation based on an Early Christian symbol which included a small cross for the eye or the Greek letters "ΙΧΘΥΣ".