The game is billed as the richest in world football, with at least 170 million pounds ($248.59 million) in future earnings for the winners, and with the stakes so high both Yorkshire teams put safety first before a crowd of just over 70,000.
It was always going to take something special to beat Wednesday's in-form Ireland keeper Keiren Westwood and Diame, who had hit the post just before halftime, produced it after 72 minutes with a superb shot which curled under the crossbar.
“Tonight we will enjoy it and celebrate,” said the 28-year-old Senegal midfielder Diame, who was man of the match and added that the goal was the most important of his 11-year career.
It was a strike, set up by Robert Snodgrass's lay-off just outside the area, that recalled memories of Dean Windass's stunner to secure Hull's first playoff triumph in 2008.
Wednesday, who have been out of the top flight since 2000, finished with four strikers on the pitch as they chased an equaliser but Hull held on to secure manager Steve Bruce his fourth promotion from the Championship.
“It was a wonder strike,” said much-travelled former Manchester United defender Bruce. “Diame has had his critics this season but it was a bit of magic that won us the match.
“I thought we did enough to win it. My big players performed excellently.”
It was also a memorable day for Hull's owner, Egyptian-born businessman Assem Allam, though he was not there to witness it.
The controversial 76-year-old multi-millionaire, who has angered many fans by wanting to change the team's name, has been ill for some time and has hardly been seen at the club.
Hull will join second division champions Burnley and runners-up Middlesbrough in the Premier League next season. Hull finished fourth in the Championship standings, nine points ahead of Carlos Carvalhal's Wednesday who came sixth.
Original source: Diame fires Hull straight back to Premier League