Fifth in the Prix de l'Arc de Triomphe 13 days earlier, Sealiway bettered the form he showed when runner-up to St Mark's Basilica in the French Derby to lift the mile-and-a-quarter showpiece.
The three-year-old colt, trained by Cedric Rossi and ridden by Mickael Barzalona, was made to fight all the way by the William Haggas-trained Dubai Honour, who only went down by three-quarters of a length.
Mac Swiney ran a big race in third place, a length and a half away. Mishriff was a length back in fourth.
Addeybb, successful 12 months ago, disputed matters, although he was a spent force early in the straight after Derby hero Adayar set sail for home.
But as in the Arc, his bid fizzled out and Sealiway came through to lead. Mishriff tried to mount a challenge, but he had no more to give in the closing stages.
It was left to Dubai Honour to put it up to Sealiway - but the French raider was just too strong despite drifting in the closing stages.
Barzalona - who won the Derby in 2011 with Pour Moi - said: "It's great to be back in the big time and be with this horse since the beginning. He deserved to prove his talent like he did today.
"I didn't sit on him on Arc day but the team were very confident, they said he was in good form and that he had improved a lot since the Arc run so although it was a tough race they were pretty confident.
"I thought I was always going to hold Dubai Honour when he didn't pass me straight away."
Rossi said through a translator: "He recovered really well from the Arc because that was a mid-seasonal reappearance and we came here with the thoughts of winning it.
"The Arc did him the world of good and he was ready for this race. All we needed to do was to keep him ticking over.
"Ascot is very beautiful. We are very proud. It's a beautiful racecourse, it's a beautiful day, beautiful racing, so very proud."
Haggas said Dubai Honour, who had won the Prix Dollar on Arc weekend: "He was a little bit unfortunate because he's a hold-up horse drawn in stall 10 and Adayar missed the break and just as James (Doyle) was trying to get him in, Adayar went hurtling past him and set him alight a bit.
"No excuse, we were quite far back but that's the way he needs to be ridden. He came with what looked like a winning run, but the other horse outstayed him.
"He's another that has made great progress. I was thinking that the Hong Kong Cup might suit him, he would enjoy that long straight and he seems to run well right-handed. I don't know about Australia for him yet."
Adayar was fifth, and trainer Charlie Appleby said: "William (Buick) said that he was always doing enough and although one back and one off the rail looked like he was in a nice position, Will just said that he was always doing too much and it paid up the straight. Those exertions paid towards the end.
"We made the decision to come here and maybe it was a tougher race than we thought in the Arc.
"We'll put him away now and he's still a nice horse for next year."