The linguistic behavior of Frederick II (1712-1786), the most significant and simultaneously controversial Prussian king of the 18th century, has been the subject of contentious and sometimes curious judgments in extensive historiography. His ostentatious preference for French was met with misunderstanding, while his competence in this language was simultaneously questioned. Some authors even went so far as to claim that he hardly knew German. This philological study provides a comprehensive representation of the monarch's bilingualism based on his manuscripts and contemporary testimonies. It examines the linguistic behavior of the French-speaking and writing private individual, writer, and philosopher, as well as the predominantly German-speaking administrator and military figure. His French orthography and some regressions are analyzed according to linguistic standards.