The total audience, across its digital and TV platforms, was 11.3 million viewers, a third higher than the audience for last year's final between Liverpool and Real Madrid when 8.5 million fans watched.
BT Sport broadcast the final to its TV subscribers as normal but it also made the game available for free via its mobile app, website and YouTube channel, with 6.5 million watching on television and 4.8 million across the digital platforms.
This means BT Sport had more viewers during the game, which kicked off at 8pm, than BBC One and ITV1 combined.
The broadcaster, which has spent £2.1billion on the rights for UEFA's club competitions between 2015 and 2021, also attracted an audience of 6.3 million for last week's Europa League final between Arsenal and Chelsea, with 3.2 million watching on TV and 3.1 million on digital platforms.
In a statement, BT marketing director Pete Oliver said: "This season's Champions League and Europa League have been a great success for BT.
"Both finals were of huge interest with four English teams competing, which has never been done before in the history of both tournaments and came at the end of a season featuring incredibly entertaining matches that captivated the nation."
BT Sport has also invested heavily in Premier League football and last year secured the live rights for 52 games a season from 2019/20 to 2021/22, an increase of 10 games on the package it offered for the last three seasons.
The company, however, has recently lost its FA Cup rights to ITV.
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