Matt Reeves, who directed 2014's "Dawn of the Planet of the Apes", has returned as the helmer for the project, set to hit theaters on July 14, 2017, said The Hollywood Reporter.
Mark Bomback is back as screenwriter, with Chernin Entertainment producing.
The storyline is being kept under wraps. Zahn will play an ape and Harrelson will play a character called the Colonel.
Zahn, 47, is known for his roles in "That Thing You Do", "Diary of a Wimpy Kid" and "The Dallas Buyers Club".
The first movie in the original franchise was " Planet of the Apes" in 1968.