An average 8.1 million people tuned in to watch the TV talent search as it was broadcast, compared with an average 4.5 million who watched The Voice.
Viewing peaked at 9.6 million on ITV as 13-year-old Beau Dermott wowed the judges with her singing.
Some 6.1 million saw Kevin Simm crowned the winner of the BBC's singing show.
Despite winning the ratings battle on Saturday, the Britain's Got Talent opener received the lowest viewing figures of any launch show since its very first episode in 2007 when 4.9 million people tuned in.
Last year 10.1 million watched the launch, down from a high of 11.1 million in 2014.
Saturday's episode saw Dermott secure a standing ovation and the first Golden Buzzer from the judges - which gives her an automatic place in the live shows - following her rendition of Defying Gravity from the musical Wicked.
After a slight fall in viewership for the following acts, audiences peaked again as 9.5 million watched Moldovan Alexandr Magala's sword swallowing act.
The judges and crowd screamed and covered their eyes as the 26-year-old slid upside-down a pole with the sword lodged in his mouth, coming to a stop an inch above the ground.
"I actually thought you were going to die," Simon Cowell admitted after the act was put through to the next round of the competition.
According to overnight figures, around 40% of all TV audiences were watching Britain's Got Talent between 19:00 BST and 20:20 BST.
When the programme finished, two million viewers switched over to The Voice for its final 40 minutes and to see the winner announced.
Simm, who was previously in the pop group Liberty X, beat runner-up Jolan to be crowned the last winner of the singing contest on BBC One before the show moves to ITV.
Although audience figures for the latest series of The Voice had started strongly with an average 7.9 million viewers, ratings steadily declined in past weeks with 4.5 million tuning into the semi-final last week.
The finale drew its lowest ever ratings, down from 6.3 million viewers who tuned in to watch it last year, and 6.6 million in 2014.

Mr Liu's family had been informed of the situation, medical staff in the north-eastern city of Shenyang said.
The dissident was diagnosed with late-stage liver cancer in May.
He was imprisoned in 2009 on charges of inciting subversion against the state after he helped to write a petition calling for political reform in China.
"Liu Xiaobo's liver function has worsened, his bilirubin levels are gradually rising," the hospital said in a statement.
Bilirubin is produced by the liver, and high levels of it can indicate liver failure.
A photograph showing the dissident looking frail was shared among friends and fellow activists on Thursday. He is standing beside his wife, Liu Xia.
In an open letter to the authorities, his friends have asked to be allowed to visit him on a "humanitarian basis".
"We feel this is deeply tragic and realise that Liu Xiaobo has few days left and fear he is near death," it says.
"At this moment, we urgently need to go to visit him to bring to Liu Xiaobo and Liu Xia their friends' care and well wishes."
On Wednesday, China invited medical experts from the US and Germany to help treat Mr Liu. Authorities said the decision was made at the request of his family.
The decision came ahead of the G20 meeting in Hamburg, Germany.
Diplomatic sources in Beijing say China has been nervous the issue might overshadow President Xi Jinping's appearance at the summit, Reuters reports.
Mr Liu, who was a key leader in the Tiananmen Square protests in 1989, won the Nobel peace prize in 2010.
His wife Liu Xia has been under house arrest since then - but she has never been charged.
Mr Liu has three years left to serve of his sentence for "inciting subversion" after drafting Charter 08 - which called for multi-party democracy and respect for human rights in China.
Amnesty International has said he should never have been jailed.
The return of Britain's Got Talent was a ratings winner for ITV, scoring almost double the viewers as the final of The Voice on BBC One.
[NEXT_CONCEPT]
The Chinese hospital treating imprisoned Nobel Peace laureate Liu Xiaobo for advanced liver cancer says his condition has worsened.