"The Turkish Armed Forces, together with the Syrian National Army (rebel groups backed by Ankara), just launched #OperationPeaceSpring," Erdogan wrote on Twitter in English.
He said the offensive targeted Kurdish militants and the Islamic State group in northern Syria.
"Our mission is to prevent the creation of a terror corridor across our southern border, and to bring peace to the area," he wrote.
Turkey has long been planning military action against Kurdish forces in northern Syria due to their ties with the Kurdistan Workers' Party (PKK), which has fought a bloody insurgency against the Turkish state since 1984.