The 37-year-old said he is looking forward to helping the Scorchers return to the top of the BBL table.
“The club has had a lot of success, and I am excited at the prospect of helping the team deliver the members and fans another championship.
“With a lot of great talent on the list and the opportunity to play at Optus Stadium, I am really excited to play for such a great club.
“Brad Hogg showed us all how useful leg spin can be in the West and I can’t wait for the season to start.”
Spinning all-rounder Agar has extended his stay with the Scorchers until the end of the 2021-22 season.
Scorchers Head Coach Adam Voges said locking down the spin options for his side was a crucial part in finalising the squad.
“Having quality spinners in your team has become such a vital part of T20 cricket,” Voges said.
“Fawad has played T20 cricket all over the world, he brings a wealth of experience and will add great variety to our attack.
“We’ve all seen what Ashton can do with both bat and ball, I’m extremely pleased he’ll be wearing orange for the next three seasons.”