The 26-year-old joined the Saddlers from Birmingham in January 2019 on a two-and-a-half year deal and was relegated to League Two at the end of that season.
In the last two seasons in the fourth tier the centre-back has been a mainstay in the back-line. He made 39 appearances last season, scoring four goals.
His contract with Walsall expires on June 30 but he was one of three players offered a new contract.
Talks were ongoing until Argyle came calling and the defender has now turned down terms at the Banks’s Stadium in favour of a move to League One.
Overall, Scarr played 96 times during his spell at Walsall and notched seven goals during that time.
A Saddlers statement read: “We would like to thank Dan for his efforts during his time at the club and wish him well for the future.”
Walsall now only have player-coach Mat Sadler and youngster Tom Leak as contracted defenders. Hayden White and Danny Coogan are still in negotiation over their new offers.
Meanwhile, Plymouth boss Ryan Lowe has revealed they almost signed Scarr in January as he expresses his delight at picking him up on a free transfer.
Lowe told Plymouth Live: "Dan is a dominant centre-back. We were close to potentially pursuing it in January but obviously he was still under contract at Walsall and it would have cost us monies.
"We were hoping that he wouldn't sign a contract at Walsall and he didn't, hence why we have furthered our interest with it.
"He's dominant in both boxes, he gets the first contacts. His stats are up there. His data is probably one of the best in blocks and first contacts.
"And I was pleased when he asked me for a goal bonus because he wanted to get on the end of things as well.
"It's what we have needed. He's a solid and resolute defender, and goes along with James and potentially a couple more that we are going to find.
"The main priority was the back lads because we never had none and we are on with that.
"We will hopefully have one more in the next 72 hours maybe and that will be three solid defenders who we feel can compete in this league and keep the back door shut, along with a top class goalkeeper in Michael Cooper.
"Ultimately, their main job is to keep the ball out of the back of the net at the other end. That's what Dan and James will certainly bring, and whoever else comes in.
"We know there is still a little bit of work to do with Dan in terms of the way we play because he has never really played a three at the back.
"But he's 26, he's 6ft 2 and we will have a powerful back three in the next few days, and that's what we have wanted to do.
"Keeping the back door shut is the priority, and then if they can help at the top end by scoring goals when you have got a George Cooper and a Conor Grant putting the ball in those areas then that's always going to be a bonus too."