The 24-year-old has been training with the Saddlers this week and featured for the reserves against Leicester.
"Stefan is a very intelligent frontman. I have known him for a long time and know exactly what he is capable of," manager Richard Money told the club's official website.
"I feel that we may have got him at the right time in his career. He is a welcome addition to our squad and this deal will be beneficial to all concerned."
Moore began his career with Villa and joined QPR in 2005 before being released earlier this year.