Mantom returns to the Banks's Stadium for a second spell, having previously been with the Saddlers on loan for the final two months of last season.
The 20-year-old will go straight into the Walsall squad for the visit of Swindon in League One on Saturday.
"Sam is a player that we know very well," Saddlers manager Dean Smith told the club's official website.
"He had a really successful time with the club last season and I am delighted that we have been able to bring him back here.
"He will strengthen us and bring added quality to the midfield. He's a good passer of the ball who knows our style of play and will fit into what we are trying to achieve here extremely well."
Mantom added: "It was a no brainer to come back.
"I know what this club is all about and it was an easy decision to make once I heard that Dean Smith was interested in me."