Winger Chris Clark, who left the club for around £150,000 in January 2008, has rejoined the club on a free transfer following the Devon club's relegation.
The Dons beat off competition from Kilmarnock, who had been expected to secure the player in the summer transfer window.
The 30-year-old utility player made 107 appearances for Plymouth, scoring twice.
Dons boss Craig Brown said: "I remember him from his original time at Aberdeen and I remember that is he is a very good player.
"His availability was too good an opportunity for us to refuse so we are delighted he has committed himself to Aberdeen.
"His versatility is the main attribute he has. He is able to play in a number of different positions.
"He is a very fit guy and a very good professional. He will be a great addition to the squad."
Clark's signing is expected to be formally announced this afternoon where he may be unveiled alongside former Coventry central midfielder Isaac Osbourne.
The 25-year-old has been without a club since leaving Coventry City at the end of the season after an injury-plague few years.
However the Dons have put Osbourne through a series of stringent tests and are set to make him the club's fourth summer signing alongside Clark, Youl Mawene and former Man City goalkeeper David Gonzalez.