An average of eight million people tuned in to watch the finale as it was broadcast live, with an additional 500,000 watching on ITV+1.
Viewing peaked at 9.7 million as Army bandsman and magician Richard Jones was announced the winner.
The previous low was in 2014, when 10.7 million saw singing act Collabro win.
Jones, a lance corporal of The Household Cavalry, impressed the judges and public with his military-themed act.
"I can't believe it, thank you so much to everyone that voted - it's been an incredible experience, it means the world to me," the 25-year-old said.
The first magician to ever win the talent show, he wins a Â£250,000 prize and the opportunity to appear at the Royal Variety Performance.
Live viewing represented a 39.7% share of all TV viewing, although final consolidated figures - where all on demand and catch-up viewing is taken into account - will not be known for a week.
An ITV spokeswoman said audiences peaked at 10.5 million viewers when ITV+1 figures were added.
The finale was broadcast at the same time as the Uefa Champions League Final between Real Madrid and Atletico Madrid, which was broadcast on subscription service BT Sport and YouTube free of charge.
Last year's Britain's Got Talent final was watched by an average of 11.7 million viewers, accounting for a 46.6% audience share.  More than 17 million viewers tuned in to watch the final at its peak in 2009, when dance act Diversity famously triumphed over Susan Boyle.
The final usually counts itself as one of the most-watched programmes of the year. However, this year it is unlikely to feature in the top 10.
'Brilliantly British'
Wearing his Household Cavalry uniform, Jones's act featured a card trick which told the story of Britain's oldest living magician, Fergus Anckorn, and how he used magic to help him cope with being a prisoner of war in Singapore.
The act culminated in bringing the 97-year-old out on stage.
Receiving a standing ovation from the judges, Alesha Dixon described the performance as "poignant, noble and brilliantly British".
Speaking on Britain's Got More Talent after the main show, head judge Simon Cowell said he thought the public voted Jones the winner not just because of his skill as a magician.
"I think people rooted for him as a person," he said. "His presentation, his whole performance was immaculate - it was very patriotic and heart-warming and he deserved to win."
Fellow judge Amanda Holden added: "I'm absolutely delighted because when magic first started on Britain's Got Talent it was rubbish and after 10 years he has done such a great job."
The Army tweeted its congratulations to Jones, as well as a picture of The Band of the Household Cavalry celebrating his win.
Swing singer Wayne Woodward - who became the bookmakers' favourite late on Saturday afternoon - finished as the runner-up with his rendition of Nina Simone's Feeling Good.
Boogie Storm, a group of disco dancing Star Wars stormtroopers, were voted third.
Also among the 12 finalists was sword swallower Alex Magala, whose act had to be pre-recorded for safety reasons. It involved him sliding upside down a pole, while handcuffed, stopping centimetres above a powered chainsaw.
Other finalists included schoolgirl singers Jasmine Elcock and Beau Dermott, mother and son duo Mel and Jaime, 100 Voices of Gospel, dancers Shannon and Peter and Balance Unity, impersonator Craig Ball and dancing dog Trip Hazard.
To celebrate the show's 10 years, a number of past winners and memorable acts appeared on stage as part of a special performance.
Choreographed by Diversity's Ashley Banjo, it featured six winning acts including 2008's George Sampson, 2010's Spelbound, 2012's dancing dog Pudsey and 2013 victors Attraction.
Last year's runner-up, Jamie Raven, also appeared, although last year's winner, Jules Dwyer and her dog Matisse, were noticeably absent from the line-up.
Dwyer became embroiled in controversy last year when it emerged a stunt dog was used in the final performance instead of Matisse.
Ofcom ruled the public had been misled over the incident, but said it did not believe ITV intended to deceive viewers.
The final of this year's Britain's Got Talent drew the lowest audience in the show's 10-year history, according to overnight figures.