The channel beat PewDiePie to the landmark, having overtaken the Swedish YouTuber to become the most popular channel on YouTube earlier this year.
T-Series, which hosts Indian music videos and movie trailers, has grown quickly over the last couple of years, inadvertently boosting the subscriber numbers of PewDiePie as the two battled it out to claim the title of YouTube's top channel.
Both channels passed 90 million subscribers within hours of each other in March, though PewDiePie's six-year reign as the world's most popular YouTube channel finally ended at the end of March.
The Swedish YouTube star, whose real name is Felix Kjellberg, put out a music video acknowledging the defeat, titled 'Congratulations'.
PewDiePie currently has around 96 million subscribers.
The rise of T-Series has proved controversial among some corners of the YouTube community, with some figures seeing it as emblematic of YouTube's perceived shift away from independent creators to focus on corporate brands.
PewDiePie previously said he "didn't really care" about T-Series, though went on to enlist his legions of fans to help keep his subscriber numbers ahead of T-Series.
"It's already a heated subject not that a company is taking over," he said in a video posted to his channel in October.
"Everyone is going on the rant, 'YouTube is not really becoming YouTube' and, 'it's never going to be the same now'... I think if YouTube does shift in a way where it does feel more corporate, [then] something else will take its place."
Among those that joined the #SubscribeToPewDiePie campaign were Tesla and SpaceX founder Elon Musk, who finally delivered on his promise to host the channel's 'meme review'.
But despite their efforts, PewDiePie was unable to hold off T-Series, which has been gaining around 1 million new subscribers each week - and that is just its primary channel.
T-Series has 25 other channels across YouTube alone, ranging from kids content to comedy, which have seen over 13 billion views in the last 90 days alone - 3 billion more than PewDiePie delivered in the last three years.
"Reaching 100 million subscribers is a big coup for T-Series and other major players in digital media," Denis Crushell, a managing director at video analytics platform Tubular Labs, told The Independent.
"We're starting to understand the volume of viewership YouTube can deliver... Media companies will try to replicate this, but we have to put into context PewDiePie's performance in the last six months - 3.9 billion views demonstrate that influencers and personalities will continue to succeed alongside media content."
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