The 25-year-old, who hails from Wageningen in the Netherlands, shared a video titled "I'm coming out" on Monday evening.
In it, she explains her decision to reveal intimate details about her past was forced by someone attempting to "blackmail" and publicly out her.
While it is unknown as of yet who the potential blackmailer was, fans of the NikkieTutorials profile have suggested that another beauty influencer could be behind it.
2019 saw a number of YouTubers – particularly make-up artists – publicly quarrel over a myriad of reasons.
Nikkie has been sharing make-up tutorials and reviews to a captive audience since 2008 and boasts almost 13 million subscribers.
Her success has seen her collaborate with celebrities including Lady Gaga and named as the first global artistry adviser for Marc Jacobs beauty in January 2019.
"Today I am here to share something with you that I've always wanted to share with you one day, but under my own circumstances, and it looks like that chance has been taken away from me," she says in the video.
"So today, I am taking back my own power and I have to tell you something."
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She continues to explain that she always wanted to be her own person -- without any labels. "It's time to let go and be truly free," she then says. "When I was younger I was born in the wrong body, which means that I am transgender."
"I am NikkieTutorials, and I am Nikkie. I am me. We don't need labels. But if we are going to put a label on it, yes, I am transgender. But at the end of the day I am me," she said.
De Jager then opened up about her childhood, explaining how she always identified as a woman.
"By the age of seven or eight I fully wore girl's clothes only and it felt amazing."
As for the timing of the decision to reveal the news today, the YouTuber said it wasn't entirely of her own choosing.
"I have been blackmailed by people that wanted to leak my story to the press," she said.
Nikkie says she always wanted to open up about this part of her life but wanted her channel to be about her art. "The last thing I want in my life is for you to not trust me any more or to look at me with different eyes," she says.
"At the end of the day, I am still Nikkie."
At one point she addresses the people she says are threatening her and makes a middle-finger gesture, telling them: "This one's for you"
She also expresses her hope that her video may help and inspire others.
"If you feel like you're trapped and there's no way out, know that it gets better. Trust me, it gets better," she says towards the end of the clip.
A number of Dutch politicians have since praised the YouTube star, with Minister for Education, Culture and Science Ingrid van Engelshoven describing her as "brave" and a "role model".
"You're truly free if you can be who you are. Role models are of great importance for emancipation," she wrote (in Dutch) in a tweet.
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