Striker Long joined on a four-year deal from Barclays Premier League rivals Hull for a fee which Press Association Sport understands could rise to £12million
Gardos also agreed a four-year contract at St Mary's, Southampton announced on their website, with the centre-back joining for a fee believed to be £6million from Romanian champions Steaua Bucharest
It has been a summer of upheaval on the south coast with the departure of boss Mauricio Pochettino to Tottenham topping off a host of big name exits.
Striker Rickie Lambert, midfield talisman Adam Lallana and defender Dejan Lovren all left for Liverpool while full-backs Luke Shaw and Calum Chambers departed for Manchester United and Arsenal respectively
But Long and Gardos join striker Graziano Pelle, winger Dusan Tadic, goalkeeper Fraser Forster, full-back Ryan Bertrand and midfielder Saphir Taider in joining the club as new boss Ronald Koeman engineers a first-team rebuild
Jay Rodriguez and Morgan Schneiderlin are both still heavily tipped to leave before the end of the transfer window but the new additions will be welcome news to the Saints faithful after a fraught few months
Long and Pelle are likely to lead the line when Southampton begin their Premier League campaign against Liverpool at the weekend and Koeman is delighted to have recruited the Republic of Ireland international
"Shane is a very good player
As a striker, he's different to Graziano Pelle, which is something we needed," Koeman told the club's official website
"He brings ambition and a great mentality
He's always running and always working, and he's very down-to-earth
He's a great person, and we need that type of player.
"He brings a lot of experience to our squad because he's already played at Premier League level for a few years
He knows what we need at this level, and he can show that to our younger players as well.
"Shane will be an important player for us and our future."
Long first moved to England with Reading and spent six years at the Madejski Stadium before moving to West Brom in the summer of 2011
He spent two-and-a-half years with the Baggies before moving to Hull in January where he scored four goals in 15 appearances.
"I'm delighted," he said
"This came out of the blue for me but, once I'd come and had a look at the training facilities and met the manager, it seemed like the right decision for me and my family
"It was a hard decision for me to make, but when you come down to Southampton you see how big a club it is and how well set up it is.
"You look at how well they did last season and they are developing a new squad for this year to try and achieve those accomplishments again, and hopefully I can be a big part of that."
Gardos comes to the Premier League after winning back-to-back Romanian titles with Steaua and Koeman believes the 25-year-old will only improve as he fills the gap left by Lovren.
"I am very happy to have Florin here," he said.
"He's still a young player so he can develop himself a lot more, and that's part of the ambition of Southampton.
"From the beginning, after Lovren left the club, we paid a lot of attention to Florin.
"Everybody knows the big names and the big players, but it's always more difficult to find one who's not so well known
The information the club has given to me about him has been very good.
"For now and for the future, we have a very good central defender."
Gardos, a 12-cap Romania international, added: "This is one of the happiest days of my life
It's the biggest step of my career so far.
"Coming to the Premier League is a very big occasion for me, and Southampton was a team that had a really good last season so this wasn't really a hard decision.
"I've had a very good first impression of the club
There's a very big difference between what I had back home and what is here.
"I've always said that coming to Premier League would be a dream come true, and luckily I've been able to achieve that dream now."
Source : PA
Source: PA