The 19-year-old centre-back has signed a five-year deal with the Bundesliga outfit and is expected to cost in the region of €22 million after Leverkusen beat off competition from Ligue 1 sides Rennes and Lyon for his signature.
“Retsos had offers from some top clubs playing internationally this year, so it is all the more pleasing for us that he has decided to move to Bayer – especially since he was so elemental to Olympiakos qualifying for the Champions League,” sporting director Rudi Voller said.
“He’s a young player with special talents and fits perfectly into our requirements. He is an investment in the future of the club.”
Retsos, who became the youngest captain of a Greek champion club aged only 18, five months and 30 days, said: “Bayer have always played in the Champions League, despite a difficult national competition, which has impressed me very much.
"To be wanted by such a club makes me proud and I’m looking forward to playing in the Bundesliga in one of the most attractive leagues in Europe.”
He could cap a remarkable few days by featuring in Greece’s World Cup qualifying matches against Estonia and Belgium – potentially his first senior international caps.