Leverkusen, who kick off their Bundesliga season against Hoffenheim on Saturday, have seen more than half a dozen players leave in the close season, including central defender Jonathan Tah, Florian Wirtz and playmaker Granit Xhaka.
They had won the domestic league and Cup double in 2024.
"In Loic Bade we have signed a young but very experienced central defender. He's used to playing against absolutely world-class strikers in LaLiga in Spain," said Leverkusen managing director sport Simon Rolfes.
"Loic is physically uncompromising and can read the game, an absolute addition to strengthening our defence. We are competitive at the highest level with this defence."
The 25-year-old France international started his career at Le Havre and also had a short loan spell at Nottingham Forest before spending two and a half seasons in Spain with Sevilla, with whom he won the Europa League in 2023.