The 23-year-old was a key target for manager Jim Goodwin who tried to sign the defender previously before he made the switch to Parkhead from Shamrock Rovers.
Scales signed a long-term deal with the Scottish champions last summer. He made 13 appearances in all competitions, scoring twice, including a fine strike in the Scottish Cup win over Raith Rovers.
A Republic of Ireland Under-21 international, he received a call-up to the full team in July 2021.
“Liam is a player that I have been trying to get for the last couple of years,” Goodwin told the club’s website.
“I watched him very closely during his time at Shamrock Rovers, unfortunately I couldn’t get a deal done.
“Liam is 23-years-old and has already amassed nearly 200 appearances in senior football. He’s a strong character with a winning mentality, who will no doubt be a great addition to the squad.
“He is coming in as a left sided centre half, which is the position I saw him play regularly for Shamrock Rovers, and the one we believe is his best position.
“He is aggressive, mobile, very comfortable on the ball and he has all the qualities you would want in a modern-day centre back.”
Aberdeen are likely to add more signings to their squad ahead of their training camp in Spain next week.