Variety was the first to report the casting news.
The Hollywood Reporter said it also confirmed Poulter's involvement in the show.
No details about his character are known at this time.
Neither Amazon nor Poulter have publicly commented on the reports.
The project was announced in 2017. Amazon said at the time that it would explore new story lines and take place before Tolkien's novel, The Fellowship of the Ring.
Jurassic World helmer J.A. Bayona is directing the first two episodes.