Outsider Auroras Encore has won the Grand National at odds of 66-1.
Winning jockey Ryan Mania tweeted only yesterday he was "worth a bet" but added that he needed the "luck of the town behind me."
Speaking to Channel 4 Racing, Mania said "There's no words to describe it, I got a dream ride all the way - I couldn't believe my luck. I couldn't fault the old horse.
"He was second in the Scottish National last year and I thought I should stay loyal to him and thank God I did. I never really had an anxious moment, he made a couple of mistakes, that's all."
Cappa Bleu, ridden by Paul Moloney came in at second place, while Teaforthree took third place and Oscar Time came in fourth.
Thousands of expectant punters had gathered at Aintree Racecourse for the sold-out 166th John Smith's Grand National.
Seabass was expected to be the best backed horse in the race, the so-called housewives' choice, given the huge media spotlight on his rider Katie Walsh who was bidding to be the first female rider to win the Grand National.
She sparked controversy ahead of the meeting when she defended the sport from accusations of cruelty and said the horses were treated better than "many children". Seabass came in 13th.
The Grand National is worth £975,000 in prize money, making it one of the richest jump races in Europe.
It will be the first time the contest is broadcast in the UK by Channel 4, after the broadcaster won the rights to show the meeting for the next four years.
Two horses have also died during this year's meeting.
Little Josh fell during yesterday's Topham Chase, which is run over the Grand National fences.
The 11-year-old, ridden by Sam Twiston-Davies, suffered a broken shoulder and was later put down by veterinary surgeons, Aintree said.
It followed the death of Battlefront after he was withdrawn during Thursday's fourth race by his jockey Katie Walsh.