The 20-year-old links up with the Saddlers having featured for United's under-23s while also having spells on the books of both Wolves and Manchester City as a youngster.
Devine spent the first half of the season on loan at Scottish club St Johnstone, making 10 appearances and featuring in both the Europa League and Europa Conference League.
The Saddlers are thought to have beaten League One and fellow League Two clubs to his signature.
"I had a few good chats with the gaffer and he said very good things," said Devine.
"I am really excited to join and give my all for the team and show the fans what I can offer. Hopefully, we can start picking up some points soon.
"It was a great experience for me (up in Scotland). It was my first loan spell and my first taste of mens football, and I thought I dealt well with it.
"Now, it is time for my next opportunity back playing in English football, which can be different, but I am looking forward to it.
"The head coach said to me that he wants me to come in and fight for my place.
"He wants me to do what I am good at and offer the team something different, and that's what I am willing to do and looking forward to doing.
"If I am going to describe myself as a player, I am an attacking full-back who loves getting up and down. I love a challenge and making things happen. I will always give my all for the team."
Devine played in a Europa League qualifier against Galatasaray for St Johnstone, and also came off the bench against LASK in the Europa Conference League.
He is expected to feature for Walsall at Bristol Rovers tomorrow.
Saddlers boss Matt Taylor added: "Reece is someone who we have been working on bringing to the club for a period of time.
“This isn’t his first loan which is very good for ourselves. He comes down from St Johnstone in the Scottish Premiership, and he is a player that I feel has the attributes to play men’s football regularly.
“He’s combative, he wants to fight, he has obviously got wonderful technical elements to his game because of his upbringing and where he has been previously as an academy player.
“The thing I like about Reece is that he was very keen on this move, and he had other options on the table and he wants to come and prove himself.
"I can’t wait for us to work with him, for his team mates to work with him and for him to understand the way we play.
“He provides us with excellent competition in an area of the pitch that we were light. We had to make sure we made good additions, and I am really pleased with being able to add Reece to the squad.”