Lewis, 19, has impressed Saddlers boss Dean Smith during his trial period and has been rewarded with a two-year deal.
The West Brom academy graduate goes straight into the squad for Saturday's season-opener against Tranmere.
"He has impressed during his time with us so far," Smith told the club's official website. "He's an up-and-coming talent who we will be able to develop and fits the bill of what we are looking for in terms of attitude, temperament and attributes."
Lewis added: "This is a good move for me. It's nice to finally have things sorted and be at a well-run local club where I feel that I can really progress.
"The target is obviously to push Richard O'Donnell as much as I can and develop. It's a good opportunity for me to prove what I am capable of."
- Sports & Recreation
- Dean Smith