Chastain is in negotiations to star as an older version of Beverly, who was first portrayed by Sophia Lillis in the first film, released in 2017, Variety reported.
Director Andy Muschietti and Bill Skarsgard as Pennywise the Clown are returning for the sequel with Gary Dauberman writing the script. Muschietti, who directed Chastain in Mama, previously said he hoped to recruit the actress for the role.
The It sequel, set to arrive in theaters on Sept. 6, 2019, will take place 27 years following the original as the Losers Club, which included Beverly, return home to the town of Derry to face Pennywise once again.
Muschietti has also said that Lillis and the other young stars that make up the Losers Club may also return for flashback scenes.
It, based on the Stephen King novel of the same name, earned $700 million worldwide -- making it the largest-grossing horror film of all time.