After spending the 2022-23 campaign on loan at Aberdeen, Scales has since become a mainstay in the Celtic side under Brendan Rodgers this term, starting 40 games in all competitions thus far.
Only goalkeeper Joe Hart (3,928) and midfielder Matt O'Riley (3.857) have played more minutes than Scales (3,630) for the Hoops so far this season, further emphasising his importance under Rodgers.
Celtic signed both Maik Nawrocki and Gustaf Lagerbielke last summer following the departure of Carl Starfelt, but Scales has successfully established himself as a first-team regular in the heart of defence alongside Cameron Carter-Vickers.
The 25-year-old centre-back joined Celtic back in August 2021 from Irish outfit Shamrock Rovers and made his first-team debut in a Scottish League Cup clash at home to Raith Rovers the following month.
Scales, a three-cap Republic of Ireland international, won the Scottish Premiership and Scottish League Cup in his first season at Celtic in 2021-22, and he is hoping to end the current campaign with two more pieces of silverware.
Scales commits future to "great" club Celtic
The defender was due to see his original deal expire in May 2025, but he has now decided to commit his future to the Hoops by putting pen to paper on a new four-year contract.
"I am delighted to agree this new contract and commit my future to Celtic," Scales told the club's official website.
"I have really enjoyed working under the gaffer, and with such a great group of players this year and I look forward to doing so again in the coming seasons.
"I love being part of this great club and I want to do all I can to contribute to Celtic and bring our fans as much success as we can."
Celtic currently sit top of the Scottish Premiership table and three points ahead of rivals Rangers with four games left to play.
Scales is expected to play in the Hoops' remaining fixtures against Hearts - on Saturday - Rangers, Kilmarnock and St Mirren before gearing up for an Old Firm Scottish Cup final on May 25.