Sawyer, who made 96 appearances for the club between 2006 and 2010, will re-join the Pilgrims at the end of this month when his current contract with Leyton Orient expires.
The left-back, who turns 30 next week, has agreed a one-year deal with Argyle.
He left Home Park in 2010 and joined Bristol Rovers on a two-year deal after a loan spell at their rivals Bristol City.
Sawyer then moved to Orient, where he made 69 appearances for the London club.
He said: “Since I left, this has always been a place I wanted to come back to.
“I’ve been thinking about it quite a bit and I’ve really enjoyed my time in London but I wanted to come back and I’m lucky enough that it fell right for me.”