The 10-year-old, trained by Sandy Thomson, had been owned by his wife, Quona.
The Thompsons bought Le Mercurey and Highland Lodge the week before last year's race, with the former 12th and the latter pulled up.
Party Politics famously won the National in their pink and purple colours in 1992.
Thomson said: "Anthony Bromley (bloodstock agent) approached us at Newbury and it has now been confirmed that he has been bought by Mr and Mrs Thompson of Cheveley Park Stud.
"It was a long drive home from Newbury thinking about it, but in the end it wasn't really a difficult decision.
[(full)Watch Replay](//) Watch the closing stages of Seeyouatmidnight's prep run at Newbury
"He's been confirmed for the race and they will be the confirmed owners."
The Berwickshire handler added: "Brian Hughes definitely rides him - nothing has changed on that front."
Thomson's part in helping save the Grand National could be rewarded in the world's greatest steeplechase.
Thomson rallied to the cause when the Aintree showpiece was under threat in the 1970s, as the race has always meant a lot to his family.
He trains from Lambden Stables at Greenlaw, near Kelso, where his grandfather bred and sold MacMoffat, who was runner-up in the 1939 and 1940 renewals. And Thomson even trains one of MacMoffat's descendants, Wide Awake.
He explained: "MacMoffat was bred by my grandfather. Wide Awake, who has been running this year, is from the same family.
"I've always enjoyed the race. I sort of grew up with Red Rum. Those were my formative years and then I remember running round Fife point-to-point course to raise money to save the Grand National.
"I remember being at Aintree when John Thorne rode Spartan Missile (to win the Foxhunters'). There was nobody there compared to the crowds they have these days.
"It's amazing to see it as the spectacle it now is."
With that background, it is no wonder Thomson is looking forward to running his stable star.
"It would be unbelievable for a small stable like us to go and win the National," he said.
"There is no other race like it. You can't put it into words exactly what it would mean.
"We've just got to keep our feet on the ground."
Seeyouatmidnight has shown himself to be a smart stayer, beating leading National hope Blaklion in the 2016 Dipper Novices' Chase at Cheltenham and finishing third in that year's Scottish Grand National.
Unfortunately, in terms of the build-up to this year's race, those performances meant nothing as the rules state a horse must have a run over fences this season.
It was touch and go whether Seeyouatmidnight would get to Aintree, as he was without a start over fences this term and therefore not eligible.
His return from injury had been delayed by the bad weather on several occasions, but thankfully Thomson was able to send him to Newbury recently, where he showed his well-being by finishing third in a handicap chase over an inadequate two and a half miles.
That was his first start since he was second over hurdles a year earlier, and first over fences since the Betfair Chase in November 2016.
"I was very relieved," said Thomson.
"I've been delighted with him after his run. He came back well and I was obviously pleased with the way he ran, especially with the way he jumped over a big course.
"Danny (Cook) was delighted and he thought he was going to win, but race fitness has just caught up with him."
Thomson has resisted the temptation of trying Seeyouatmidnight over replica Aintree fences.
"We decided not to give him a pop over National-style fences," he said.
"He jumped really well round Newbury, so we didn't think we needed to do that."
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