Basque side Eibar have bolstered their squad with the additions of Dani Nieto and Borja Ekiza.
Thibaut Courtois would like to see Petr Cech remain at Chelsea so that he can benefit from the Czech's vast experience.
Mantom, 20, has impressed during his second loan spell at the League One club and has now put pen to paper on an 18-month deal.
"I am really pleased that we were able to complete this deal," said Saddlers manager Dean Smith. "Sam is a great acquisition for us. He's a young player who understands the game.
"We know all about him and what he will bring to us; he has hunger and a great attitude to progress.
"He's enjoyed his loan spells here and was keen to commit himself to our cause."