Schalke will sign Senegalese defender Salif Sane from Hannover on a four-year contract at the end of the season.
The 27-year-old has activated an opt-out clause in his deal with Hannover, which was set to run until June 2020, to make the move to Gelsenkirchen.
Schalke's director of sport Christian Heidel told his club's official website: "With Salif Sane we have found a flexible defensive player who knows the Bundesliga. We are happy he has chosen Schalke."
Sane, the younger brother of former Bordeaux centre-back Lamine Sane, said: "I am very happy to be part of Schalke as of the summer. Until then, my full focus is on the remaining games with Hannover."
Meanwhile, Schalke defender Matija Nastasic has been ruled out for the rest of the season after suffering cruciate ligament damage to his right knee.
Nastasic went off in the closing stages of Schalke's defeat to Hamburg on Saturday and the club said Thursday's diagnosis means the 25-year-old will be out for eight to 12 weeks.
"Whether he will feature for Serbia at the World Cup this summer in Russia is still unclear," Schalke said.
Nastasic has played a key role in ensuring Schalke has the joint second-best defensive record in the league this season. Only Bayern Munich, the champion, has conceded fewer goals.
Information from Press Assocation and The Associated Press was used in this report.