Saints have completed the £12.5m signing of Shane Long.
The 27-year-old striker has become the club’s second most expensive signing of all time after agreeing a four year contract to move from Hull to St Mary’s.
Saints had considered a move for Long in January, after the striker was identified by the club’s scouting and recruitment department as a somebody who would fit neatly into the team. A move has been swiftly made now after Koeman asked for another forward to be signed.
In a club statement, a delighted Koeman said: "Shane is a very good player. As a striker, he’s different to Graziano Pellè, which is something we needed.
"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 added: "I’m delighted. 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.
"Southampton play a brand of football that’s enjoyable to watch, and I’m sure it’ll be enjoyable to play in as well. I think the formation they play will suit me, and the manager has a lot of knowledge about football and I’m looking to learn off him.
"In theory it’s a very good move for me, but all the hard work starts tomorrow when I get on the training pitch because I have to prove my worth."
Long, who has 47 caps and 11 goals for the Republic of Ireland, had only been at Hull since January, when he moved to the club from West Brom.
In his short spell with the Tigers, he made 15 Premier League appearances and scored four times.
Prior to that, Long had been at West Brom since August 2011, after signing from Reading for £6.5m.
He netted 19 top-flight goals in 60 starts and 21 substitute appearances for the Baggies.
Long had previously enjoyed six good years with the Royals, who picked him up from Cork City, scoring 54 times in 203 appearances across all competitions.