Frederik (real name Ilkka Juhani Sysimetsä, born February 2, 1945) is a Finnish singer. He is known by his nickname Reetu. During his lengthy career of machostyle disco and pop music, he has been awarded five gold and three diamond records. One of his contemporary nicknames is Junttidiscon kuningas (King of hillbillydisco).

Some of his best-known songs are Jos jotain yrittää (Harva meistä on rautaa), Linda Linda, Tsingis Khan, Sheikki Ali Hassan, Titanic, Stenka Rasin, Kasakkapartio, Volga, Kung-Fu Taistelee and many more.