The 24-year-old – Forest Green's top scorer with 27 goals as they won the League Two title last year – joins until the end of the season.
Stevens' campaign ended in pain though as he suffered an anterior cruciate ligament injury in April, and only returned on Boxing Day.
And Saddlers head coach Michael Flynn hopes to 'get him up and firing' again.
"I'm over the moon," said the boss. "He's shown real hunger and a desire to come and play for us. He could have took the easy option and moved to a club closer to home.
"He's always enjoyed playing at the Poundland Bescot Stadium, he's scored a few times there and now I'm hoping he does that in a Walsall shirt because if we can get him up and firing straight away then he's going to be a big signing for us."
Stevens, from Guildford, started out at Barnet before being snapped up by Peterborough United, who sent him out on loan to Sligo Rovers and Slough.
But it was at Forest Green where he really made his name despite getting off to a slow start, scoring seven goals in his first season with the club and another six in 2020/21.
But he started the 2021/22 campaign with goals in his first four games and went on to find the net 27 times in 42 appearances before suffering that season-ending ACL injury.
He finally returned on Boxing Day, making six substitute appearances since - including a couple with Jamille Matt, who also made the switch from Forest Green to Walsall this month.
"It has been tough," Stevens told the BBC, when talking about his injury four weeks ago. "Especially the way the season went last year and how well I was doing. It came towards the end of the season and luckily enough we were in a really good position as a team and we went on to win the league.
"It was tough to come out with that and have to watch the boys get us over the line instead of me being out there myself.
"It makes you a stronger person and hopefully I can only become better for it."