The show is currently in its seventh season, whose finale is slated for May 11, reported Deadline.
There is, however, no word yet on Fillion's co-star Stana Katic, whose contract also is up at the end of this season and who continues to negotiate with "Castle" producer ABC Studios.
Indicating ABC's intention to pick up another season of Castle is the fact that a search is already underway for a new showrunner.
"Castle" executive producer David Amann, who served as showrunner this season, is stepping down. He wrote three episodes this season, including the season premiere.