The social media star has more than four million followers across her combined social channels and averages 15 million views a month on her YouTube channels.
The 19-year-old joins previously announced contestants David James, Chris Ramsey and Emma Barton.
She said: "I am really excited to go on the Strictly journey and am feeling very grateful for the opportunity!
"I'm looking forward to learning all of the dances, finding out who my partner is, getting to wear the costumes and to just experience this once-in-a-lifetime opportunity."
The news about Barker was revealed on Heart Radio's Breakfast Show after footballer James, comedian Ramsey and EastEnders star Barton were unveiled on The One Show on Wednesday night.
In 2017, her book, Saffron Barker Vs Real Life, reached number one on the Sunday Times bestsellers list and earlier this year she completed the London Landmarks Half Marathon for the
Make A Wish foundation and the London Marathon for Dementia Revolution in memory of her great-grandfather Ted.
Footballer James said he had been asked to do Strictly a few times but was finally talked into it by former contestant Mark Foster.
Comedian Ramsey said he was "really buzzing" about the opportunity, joking that he had struggled to keep it a secret and confessing that he had told a few friends.
Barton, who plays Honey Mitchell in EastEnders, said she was a Strictly super-fan and that she was "over the moon" when she received the call to take part.
This year's series will see the return of judges Shirley Ballas, Craig Revel Horwood and Bruno Tonioli, while Motsi Mabuse, sister of Strictly professional Oti, will replace Dame Darcey Bussell on the panel.
Motsi was previously a dancer and then judge on Let's Dance, the German equivalent of Strictly.
The new series, fronted by regular hosts Tess Daly and Claudia Winkleman, also has a new face in the professionals line-up in the form of Latin dancer Nancy Xu.
Xu was a finalist on So You Think You Can Dance? in her home country of China and performed in hit West End and Broadway show Burn The Floor.
She takes over from Strictly professional Pasha Kovalev, who announced his departure from the show earlier this year, telling fans it was "time for me to find a new challenge".
Bookies have set odds on speculative contestants including names like Sam Allardyce, Jodie Comer, Una Healy, Michelle Visage and Chris Evans.
Radio DJ Evans revealed last year that he had spoken to Strictly show bosses but more recent reports have suggested he may not be in this year's line-up.