The 27-year-old claimed 24 Sky Bet Championship goals and 13 assists for the Saints last term as they won promotion from the second tier via the Play-Offs. Only five outfield members of the Saints squad have started more Premier League fixtures than Armstrong, who has played a pivotal part this season.
He will now link up with manager Tony Mowbray for a third time following previous successful spells with Blackburn and Coventry. Sporting Director Andrew Nestor spoke on the deal and said he is the perfect fit for the Championship side.
“We are pleased to welcome Adam to The Hawthorns for the rest of the season. He has extensive experience in the Championship and a proven goalscoring record.
“We had always planned to reinforce our striker options and injuries have accelerated our need in this area.
“Adam is an ideal fit for our squad and very much suits our established style.”