Armstrong has penned a new contract to 2027.
He told the club's website: “It’s amazing. Coming off last season, having that buzz, coming back into pre-season and getting started again brings back memories, so I’m delighted.
“I’m looking forward to it – back in the Premier League, it’s the best league in the world. It’s just about enjoying it and hopefully pushing on.”
Saints manager Russell Martin also told the club's website: “Arma has been such a huge player for us since we came to the club last summer. As someone who had lived through the pain of the previous season, it was clear he had a fire in his eyes and a determination to put that right.
“Even looking beyond his amazing numbers in terms of goals and assists, his relentless desire to press the opposition and willingness to play in different positions, sacrificing himself for his teammates, are outstanding attributes that speak volumes for his character.
“I’m delighted we’ve been able to secure his future with us, and I have no doubts he will be just as important to us in the Premier League as he was in the Championship.”