Geis, who had not featured for the Royal Blues yet this season, was able to move to the UEFA Champions League participants after the German summer transfer window had shut on 31 August as Spanish clubs have until midnight CEST on 1 September to conclude their business.
"It wasn't our plan for Geisi to leave," Schalke's sporting director Christian Heidel explained. "He came to us [on Wednesday] night and asked us to give him the opportunity to go to Sevilla for ten months. After careful consideration, we decided to grant his wish."
"I'm very grateful to Schalke for giving me the opportunity to join Sevilla," Geis added.
The 24-year-old midfielder moved to Gelsenkirchen from Mainz in 2015, making 28 Bundesliga appearances in an impressive debut season before playing 18 top-flight matches last term.
Geis joins former club captain Benedikt Höwedes in leaving the club on loan, after the World Cup winner joined Juventus on temporarily earlier in the week.
Watch: A tribute to another Schalke man departed, Benedikt Höwedes