The 27-year-old made 26 competitive appearances for French club Marseille last season, and played regularly for Argentina on their way to reaching the 2026 FIFA World Cup final.
Predominantly a left-back, Medina also provides defensive cover in central positions, and Leverkusen's second defensive signing after Miguel Gutierrez, who joined from Napoli.
Leverkusen sporting director Simon Rolfes said: "In Facundo Medina, we have signed a highly experienced professional who, thanks to his quality and strong mentality, will immediately be a key player for our team.
"He's a player who excels in one-on-one situations: assertive and aggressive. Facundo's qualities add an important element to our squad's profile. We're delighted to have him in our ranks."
Medina played four games at the 2026 FIFA World Cup, including coming on as a substitute in the final, as Argentina finished runners-up to Spain following a 1-0 defeat.
"Making the move to Bayer 04 in the Bundesliga is fantastic for me," said Medina. "At the World Cup in North America, Exequiel Palacios told me a lot about the club and spoke very highly of Leverkusen.
"Pala has been here for so long and has enjoyed great success – that's something I'd like to experience with Bayer 04 too."
Aston Villa are pushing to complete a deal for Joao Palhinha and have stepped up their efforts to sign the Bayern Munich midfielder, following his stint at Tottenham last season.
Bayern Munich have completed the permanent signing of Senegal midfielder Bara Sapoko Ndiaye from Gambinos Stars, with the 18-year-old signing a contract until June 2031.
Sead Kolasinac experienced a nostalgic return to the VELTINS-Arena on Saturday, lining up for Atalanta against his former club Schalke 04 in a friendly.
RB Leipzig captain David Raum is set to stay with the club through at least the 2026/27 season, though his long-term future remains uncertain.
