After a lengthy and protracted contract saga, Celtic finally landed Dundee United midfielder Gary Mackay-Steven late on deadline day.
The Scottish midfielder, 22, had become embroiled in an on-off switch during the final day of the winter transfer window, with some reports suggesting that Celtic had given up on their hopes of signing Armstrong.
However, late into the day, the Bhoys did finally snatch Armstrong away from their Scottish Premiership rivals, handing the midfielder a three-and-a-half year deal at Celtic Park.
It's a huge addition for Celtic and a real injection of quality to the centre of Ronny Deila's midfield, and fans were unsurprisingly ectstatic with his arrival.
Fans tweeted about how they would have been gutted to have missed out on Armstrong's signature after such a prolonged chase for his services, with many believing that he is one of the top Scottish talents in the game right now.
However, others believe that Armstrong's arrival means that Kris Commons will leave the club when his contract expires this summer.
Here are some of the reactions from Celtic fans after Armstrong finally put pen to paper at Celtic Park...
Delighted we have signed Armstrong and GMS tonight #Celtic— Daniel Kelly (@DanielKelly67) February 2, 2015
Armstrong and GMS signed, brilliant work by Celtic. #Celtic— John McNeil (@JohnMcNeil15) February 2, 2015
Commons defos won't be signing a new deal now that Armstrong is coming in. Gutted but a class signing from deila— Mark McKenna (@markmck6) February 3, 2015
Would've been gutted if we didn't get Armstrong Amazing signing— mark canavan (@canavanno) February 2, 2015
Delighted with the signing of Stuart Armstrong!— Alan Simpson (@AlanCFC) February 2, 2015
Absolutely buzzing with this Armstrong signing. Best non celtic talent in any of the Scottish leagues for me— Bradley Abbott (@bradleyabbott_) February 2, 2015
Take back what I said about Armstrong he will be a good signing but I really think he should have stayed at united until the summer— Rosssss (@gameoverrr__) February 2, 2015
Delighted with Celtic signing Armstrong would've been such a miss if we didn't get him— Kieran Hughes (@khughes17_) February 2, 2015
I said it a few weeks back that the hoops shud sign Stuart Armstrong and they have great signing. He's gonna be a top player #prospect— Martin Grehan (@MGrehan84) February 2, 2015
Actually buzzing about Armstrong signing for celtic guys a class act— Sean Daniel Levy (@SeanLevy1994) February 2, 2015