The Saints confirmed the transfer on Tuesday
Southampton have confirmed the signing of Stuart Armstrong from Celtic on a four-year deal.
The Scotland international has joined the Premier League side for a fee believed to be in the region of £7million.
Armstrong’s future at Celtic Park had been in doubt after negotiations over a new contract had stalled, leaving Brendan Rodgers to cash in with just a year to run on his current deal.
The former Dundee United midfielder was part of the Celtic side that won the ‘double treble’ under Rodgers and won a total of eight major trophies during his time at the club.
He scored 17 times in 2016-17 and forced his way into the Scotland setup, but struggled to replicate that form last season after an injury-disrupted campaign.
Armstrong told the club website: “Southampton seems a really good fit for me. Watching from afar, I really enjoy the attractive football that they play and I’m very happy to be here.
“Every season, I put demands on myself to create and score goals. I hope going forward from here that it will be the same this term at St Mary’s.”