The 16-year-old is said to have struck up a romance with Chalamet after meeting him at the Fiorello H LaGuardia High School of Music & Art and Performing Arts in New York, where they are both pupils, reported New York Post.
"It's common knowledge at the school that the pair are dating," a source said.
Chalamet plays Vice President William Walden's son Finn in the hit US drama "Homeland".