Daley, 21, and Reid, who turned 20 on Monday, topped the podium in the mixed three-metre synchro with a final score of 321.06 at the Queen Elizabeth Olympic Park Aquatics Centre.
The duo beat Italians Maicol Verzotto and Tania Cagnotto into second place, with Russia’s Nadezhda Bazhina and Nikita Shleikher collecting bronze.
Daley, from Plymouth, and Reid had only begun training together a week before competition, but they were in the podium positions throughout.
They moved into a narrow lead in the penultimate round of dives and a score of 75.60 from their final effort secured Reid her maiden major championship title and Daley his third European gold.
“I don’t really know what just happened,” Daley said.
“Grace and I were put together 10 days ago and we’ve done about five sessions together so I’m over the moon to come away as European champions on a board that I never train on.
“It could be a possible event at 2020 in the Olympic Games and Great Britain are really strong at the mixed synchro events.”
Reid had finished seventh in the individual 1m springboard earlier in the session.
She said: “Today was a really good day for me and I don’t think I’m going to sleep tonight.
“There’s a lot to take away from today and hopefully I can continue to build on it this season.”