Phrynichus of Bithynia was a grammarian and lexicographer who lived in the 2nd century CE, during the Roman Imperial period. He was a student of the famous orator Herodes Atticus and was part of the Atticist movement, a scholarly effort to revive the classical Athenian form of Greek.

His primary work was a lexicon titled Selection of Attic Verbs and Nouns, which survives in an abridged form. He also wrote a handbook for orators called Sophistic Preparation, which is now mostly lost. In his writings, Phrynichus did not just list vocabulary; he actively argued for using pure Attic Greek and against using the common Greek of his own time, often labeling non-classical words as vulgar or incorrect. According to modern scholars, his work represents a strict, prescriptive side of the Atticist movement. While his extreme purism was debated even in antiquity, his lexicon provides valuable insight into the language debates of his era and was used by later Byzantine scholars.