Karius has been the subject of much speculation in recent weeks, with a number of European clubs thought to be interested in his services.
Arsenal, Valencia, as well as Mainz's Bundesliga rivals Borussia Dortmund and Schalke, have all been said to be keen on luring Karius away from the Coface Arena.
But Karius has pledged his future to Mainz, for whom he has made 41 Bundesliga appearances.
He will now try to help transform the fortunes of a Mainz side that sits 12th in the German top flight following a run of nine games without a win.
Mainz return to Bundesliga action on January 31 when they host Paderborn.
‘The pure simplicity of the way Slot has managed the squad is probably the biggest thing I could say about him. It’s not broken, so let’s get on with it’: Liverpool legend full of admiration for Jurgen Klopp's successor at Anfield
‘I trained at Spurs and thought they’d give me a chance. But I received a letter thanking me but saying they didn’t think I was good enough – I was gutted’: How Tottenham missed out on signing Wales legend John Toshack
