And the Scottish international insists the club are heading in the right direction ahead of their first game of the New Year against Liverpool at St. Mary’s.
Armstrong, 28, has established himself as a key player in Ralph Hasenhuttl’s side and has made 79 appearances for the Saints since joining from Celtic in 2018.
'I'm very happy'
He told southamptonfc.com: “I’m very happy. It’s a great group, we’re doing well, it’s very positive and to extend my time here and be part of the club going into the future is something I’m very happy about.
“It’s a great club to be a part of and it’s a club that has a feel of positivity and heading in the right direction.”
Armstrong is the latest in a string of first team players to sign new deals in recent months, following on from the likes of James Ward-Prowse, Oriol Romeu, Jarek Bednarek and Shane Long, as well as manager Hasenhuttl himself.
And the Saints boss expressed his delight in seeing Armstrong commit his future to the south coast club and also stressed his importance to the team.
'Important for our team'
Hasenhuttl told southamptonfc.com: “I am very happy with this news. Stuart is a very smart player and is someone who is important for our team.
“He has become more comfortable and confident playing in the number 10 position, and it has been pleasing to see the development he has made as a result of the work he has done with us on this.
“I think he is someone who can continue to improve and make a very positive contribution for us here in the future, so it is good for the club and for him also to sign this deal.”