An average of 10.9 million viewers watched Ariana Grande, Katy Perry, Miley Cyrus, Liam Gallagher, Pharrell Williams, Coldplay and the Black Eyed Peas take to the stage.
The emotional concert, which aired on BBC One, attracted a peak of 14.5 million viewers.
It was also streamed simultaneously across the world on several channels and on Twitter.
One Love Manchester concert becomes most-watched tv show of 2017 so far
I’m so honoured to be at this incredible event, surrounded by all you amazing people
Miley Cyrus, after joining Pharrell Williams for a surprise duet of Happy, said: “I’m so honoured to be at this incredible event, surrounded by all you amazing people.
“For me, the most important responsibility we have on this entire planet is to take care of one another – and look what we’re doing today and how amazing that is.
“As humans we should always be who we say we are. Ariana I think has proved that.
The emotional concert, which aired on BBC One, attracted a peak of 14.5 million viewers One Love Manchester concert Mon, June 5, 2017
One Love Manchester: Ariana Grande puts on an epic show with famous faces including, Miley Cyrus, Katy Perry, Coldplay and Liam Gallagher.
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Ariana Grande performs on stage during the One Love Manchester Benefit Concert
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“She says she’s a good role model for so many girls and she has proved that by putting this together and allowing all of us to be a part of it.
“But tonight, I am here for all you.”
She added: “I have always loved Manchester, it’s always felt like a home away from home for me.”
Robbie Williams was emotional as he took to the stage, leading the crowd in moving renditions of Strong and Angels.
Robbie Williams was emotional as he took to the stage
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Britain's Got Talent, on ITV, held the previous record for the year with 9.4 million viewers on May 6.
Sunday's concert, to honour and raise funds for the victims and families affected by the Manchester attack, aired to over 50 countries, including Brazil, the US, Germany, China, the Caribbean Islands and Australia.
According to official consolidated figures – which includes catch-up and other services – the most-watched programme of the year was the Broadchurch finale, broadcast in April, with 11.6 million viewers.
Consolidated figures for One Love Manchester have not yet been revealed.