Critolaus of Phaselis was a Greek philosopher of the 2nd century BCE. He was a leading member and head of the Peripatetic school in Athens around the middle of that century, following the teachings of Aristotle.

He is most historically notable for being part of the famous Philosophers' Embassy sent from Athens to Rome in 155 BCE. The delegation, which included prominent Stoic and Academic philosophers, aimed to appeal a political fine. Their speeches introduced Roman audiences to advanced Greek philosophical debate and caused a significant cultural stir.

None of Critolaus's own writings survive intact; they are known only through fragments and reports by later authors. He is said to have written treatises on topics such as the soul, ethics, and physics. His significance lies in his role as a major defender of Aristotelian thought during a period when the Peripatetic school was less influential. In ethics, he argued that virtue was of incomparable worth compared to other goods. In physics, he upheld the Aristotelian view of an eternal world against rival Stoic theories.

Academics regard him as the most important head of the Peripatetic school between the 3rd and 1st centuries BCE. His embassy to Rome marks a key moment in the transmission of Greek philosophy to the Roman world.