The left-arm quick has been recruited as a replacement for fellow Englishman Brydon Carse, who was ruled out of the tournament by a knee injury.
Mills flew to Australia from the UAE, where he has been involved in the T10 League, on Monday and will join up with the Scorchers after a period of quarantine.
The 29-year-old joins former Sussex batsman Laurie Evans and New Zealander Colin Munro on the outfit’s overseas roster.
This will be Mills’ third stint in the Big Bash having previously represented Brisbane Heat in 2016/17 and Hobart Hurricanes in 2017/18.
Speaking to the Perth Scorchers website, he said: “When they got in touch, I was extremely keen. They’re a great franchise with some great players.
“It’s exciting to get the opportunity to play in big competitions, and when you join a new franchise, it’s a great chance to meet new people and get in with a great group of guys.
“I had a season with Hobart a few years ago when, personally, I didn’t bowl as well as I know I could have.
“I’m looking forward to showing a better side of my cricket and getting amongst the lads. It’s always good to join in with a new group, make some new mates for the next couple of months, and be successful. We all play cricket to win tournaments. I can’t wait to get over and get started.”
Scorchers Head Coach, Adam Voges, added: “We’re really excited about getting Tymal in after Brydon’s unfortunate injury.
"We saw the impact he had for England over in the World Cup and his last 12-24 months have been outstanding. He’s a world-class performer, and he’ll obviously be really important for us at the back end of the innings. In the death overs, I can see him and AJ Tye being a really good combination.”
The 11th edition of the BBL started last Sunday (Dec 5).