The 22-year-old took to Twitter to celebrate his move to Celtic but also thanked his former club.
Stuart Armstrong has tweeted his delight at joining Scottish champions Celtic.
But the 22-year-old had not forgotten what former club Dundee United have done for him during his career.
Indeed, Armstrong, who joined Celtic on Deadline Day - along with fellow team-mate Gary Mackay-Steven - is grateful to Dundee United staff, players and supporters alike.
The midfielder, however, is now looking forward to the next chapter in his career with the Bhoys.
Thank you to the club that gave me everything and allowed me to fulfil my dream of becoming a professional footballer. #DUFC— Stuart Armstrong (@StuArmstrong16) February 5, 2015
To all the supporters, staff and players, thank you for all the memories that have made my time at the club so special. I'm forever grateful— Stuart Armstrong (@StuArmstrong16) February 5, 2015
I am delighted to have signed for Celtic and to be part of such a massive club. Can't wait for all that lies ahead!— Stuart Armstrong (@StuArmstrong16) February 5, 2015
Armstrong recently denied that both he and Mackay-Steven left Dundee United as they pushed through a move to Celtic, instead suggesting that the clubs had more to do with forcing a deal - according to BBC Sport.
Ronny Deila's side currently sit top of the Scottish Premiership but will turn their focus to the Scottish Cup this weekend, when they travel to Dundee.
Dundee United, meanwhile, who sit just six points behind Celtic in the top flight, will face Stranraer in the cup on Sunday afternoon.
Armstrong spent five years at United and joined the Hoops during a Deadline-Day deal. The 22-year-old has a number of caps for the Scottish Under-21 international side but is yet to represent the senior team.