Shane Long has completed his switch from Hull City to Southampton for an undisclosed fee, believed to be £12.5m.
The St Mary's club announced the four-year deal on Thursday afternoon.
The 27-year-old's eight-month stay at the Tigers has ended with City set to pocket a profit of just over £5m from the sale.
Long's former club West Bromwich Albion will pick up £600,000 as part of a sell-on clause.
The fee eclipses City's record sale of £4m paid by Sunderland for Michael Turner five years ago.
City paid £6.5m to buy Long from West Brom in the January transfer window. His four goals in 15 games helped the Tigers stay in the Premier League.
Long travelled to St Mary's on Wednesday to discuss personal terms and undergo a medical, with the formalities of his new contract concluded today to allow him to feature in Saints' opening match at Liverpool on Sunday.
The Irish international said the move had come "out of the blue".
Speaking to Southampton's official website, Long said: "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."
Earlier, Steve Bruce told the Mail the club had not been looking to sell Long but had decided to accept a "fantastic" offer. The money will now be reinvested in strengthening the squad.
"The challenge is always to become bigger and better and this money will be put towards strengthening the squad," said Bruce. "Whether you are Christiano Ronaldo or Lionel Messi, everyone in football has got their price."
Looking forward to an exciting new challenge @SouthamptonFC . Great club with a great history. Can't wait to get going now
— shane long (@Shanelong009) August 14, 2014