The Swans had been hunting a centre-forward for months, and in Idah they have landed a 32-cap Republic of Ireland international with proven pedigree.
The 24-year-old cost an initial £6m, rising to £7m with add-ons, making him Swansea’s most expensive signing outside the Premier League era.
He has committed to a five-year contract.
Head coach Alan Sheehan said the club had finally secured the figurehead they had long desired.
“Adam's all-round ability and his experience at the highest level make him a quality addition to our squad, and his signing is an indication of our ambitions for the club moving forward,” said Sheehan.
“He is an ambitious young player who wants to play a big role in what we are trying to build at Swansea, and his movement and ability to stretch defences will be a real asset to us.”
Director of football Richard Montague echoed the sentiment, calling the striker a long-term building block.
“We have wanted to add to Alan's forward options during this window and a lot of hard work has gone in to trying to find a player who was the right person and the right fit for that role,” said Montague.
“Adam has a wealth of senior experience, including at the highest level of European club competition, and we expect him to make a considerable impact for Swansea City.”
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Idah arrives after two stints in Glasgow, initially on loan in 2023-24 before Celtic made the move permanent for around £9.5m.
He departs Parkhead with 29 goals from 76 appearances.
Before that, he came through Norwich City’s system, scoring 17 times in 116 games.
The Swans had to be patient: although a fee was agreed last week, Celtic delayed completion until the final hours of the window.
That wait proved worthwhile, with the Cork-born forward now joining a front line that also includes Zan Vipotnik, who had been linked with a departure but is staying put.
His form early this season has persuaded Swansea not to cash in.
If Idah was the headline, Wrexham’s relentless business provided the drama.
Already boasting the Championship’s highest net spend, the north Wales club splashed further cash on deadline day with three notable additions: Coventry City captain Ben Sheaf, Blackburn defender Dom Hyam and Manchester City full-back Issa Kabore on loan.
Sheaf’s £6.5m switch was the centrepiece, the 27-year-old signing a three-year deal after slipping down the pecking order at Coventry despite a reputation as one of the division’s most reliable midfielders.
He made his feelings on a move to Wrexham clear.
“The trajectory the club is on, I'm excited to be on board with that,” he said.
“From speaking to the manager, and from the ambition of the owners, it's clear it's only going one way. I'm looking forward to contributing.
“When the opportunity to move to Wrexham came up, it's something I really wanted to be able to do – I'm excited to get started now.”
Hyam, 29, cost around £2m and arrives on a two-year deal from Blackburn, where he was an ever-present last season.
Preparing for Scotland duty, he spoke of the appeal of Wrexham’s meteoric rise.
“It's really exciting for me. I'm looking forward to meeting the players and the fans and getting started,” he said.
“Speaking to people about Wrexham, and seeing how quickly it has gone from strength to strength, it is so attractive as a player to be a part of that.
“Seeing the success and how well they have done on the pitch, and how they have strengthened every season, just really attracted me to it.
“I thought it was a really exciting opportunity.”
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Kabore, 24, a Burkina Faso international with 49 caps, rounded out the spending.
A versatile right-sided defender, he arrives after loan spells at Marseille, Luton, Benfica and Werder Bremen.
“I'm feeling good, I'm very happy to be a Wrexham player,” he said.
“The project here is so amazing. Wrexham is a family club, a fantastic club, and that's why I'm here. I will give everything for this team.”
Together, the three arrivals made it 13 signings for manager Phil Parkinson in a single summer, with more than £30m invested by the Hollywood-backed club.
In contrast to the fireworks elsewhere, Cardiff City opted for restraint, but still secured a promising talent in Omari Kellyman.
The 19-year-old attacking midfielder joined on a season-long loan from Chelsea, less than a year after a £19m switch from Aston Villa.
Kellyman has endured hamstring troubles and has yet to play senior football at Stamford Bridge, but he believes Cardiff under Brian Barry-Murphy can be the right environment to relaunch his career.
“I like to make things happen. I like to be forward thinking and get on the ball as much as possible. I want to come here, get some goals and assists and help the team,” he said.
“I've been watching the recent games and it's been great to see the exciting players we already have. I'm looking forward to playing with the lads.”
The Bluebirds, competing in League One after relegation, have been unbeaten so far under their new coach, who hopes Kellyman can supply creativity in the final third and fit in alongside earlier signings Nathan Trott and Gabriel Osho.
Newport County left it late but managed to wrap up two loan signings from Leicester City before the transfer window closed, bringing in Nathan Opoku and Sammy Braybrooke.
Striker Opoku, 24, and midfielder Braybrooke, 21, will link up with Dave Hughes’ squad on deals that run until January 2026.
Braybrooke, an England Under-20 international, arrives in south Wales after gaining experience north of the border with Dundee during the 2023-24 campaign.
Taken together, the closing of the transfer window leaves Welsh football with an altered look.
Swansea have invested heavily in a focal point who could yet define their season, Wrexham have doubled down on their dramatic rise with another wave of big-money recruits, and Cardiff have opted for patience with youth.
