Ichthyas (Greek: Ἰχθύας; fl. 4th-century BCE), the son of Metallus, was a Greek philosopher and a disciple and successor of Euclid of Megara in the Megarian school. He was a colleague of Thrasymachus of Corinth in the school. Ichthyas is described as a man of great eminence, and Diogenes of Sinope is said to have addressed a dialogue to him.
- Diogenes Laërtius, ii. 112; Athenaeus, viii. 335; Suda, Euclides
- Diogenes Laërtius, ii. 113
- Diogenes Laërtius, ii. 112; cf. vi. 80
- H. Emonds, (1937), Die Oligarchenrevolte zu Megara im Jahre 375 und der Philosoph Ichthyas bei Tertullian, Apol. 46, 16. Rh. Mus. 86, 180-191.
- Tertullian, Apologeticum, 46, 16: "et Ichthyas, dum civitati insidias disponit, occiditur."