The 18-year-old actor - who is best known for playing Eleven in the Netflix series - starred in the first Enola Holmes film in 2020 and it was a big hit with viewers.
She is now reprising the role in a follow-up film, and Variety says that Netflix will pay her 10 million dollars (around £8.4 million) for her performance.
According to a tweet from Film Updates, it is thought to be the "highest upfront salary for an actor under 20".
Other young actors to have pocketed hefty salaries include Dakota Fanning, who is thought to have been making as much as four million dollars (£3.3 million) a film before she was in her teens.
And Will Smith's son Jaden is rumoured to have been paid a seven-figure number for his remake of 80s classic The Karate Kid, which was released in 2010 when the actor was 12.
Enola Holmes starred Brown as the youngest sibling in the famous Holmes family.
She heads to London to find her mum, who is missing, but ends up embroiled in an exciting adventure of her own.
The sequel was announced last year, and Brown said she was excited about playing the character again.
“I can’t wait to collaborate again with my Enola Holmes family!” she said in a statement at the time.
"Enola holds a special place in my heart – she’s strong, fearless, intelligent and brave.
"I look forward to fans seeing how her journey continues!”
The star also shared a picture on Instagram of herself in character as the detective, telling her fans on the platform: "The sequel is afoot."
Watch: Stranger Things creators respond to Millie Bobby Brown's criticism