The 45-year-old actor reacted to the news in a Twitter post Tuesday following word that Katic will leave the ABC series after Season 8. Fillion and Katic co-star as Richard Castle and Kate Beckett, respectively.
"Castle has been one of the greatest joys of my creative life, and I hope the show continues on for years to come," Fillion wrote.
"Stana has been my partner all this time, and I thank her for creating the character of Beckett who will live on for all of us as one of the greatest police officers on television," he added. "I wish her well, and have no doubt she will succeed in everything she pursues. She will be missed."
Fillion's well wishes follow rumors that Katic is leaving because of conflicts on set. Sources told Us Weekly the co-stars "completely despise each other," although Katic's rep said the star has "absolutely no issues" with Fillion.
"They will not speak when they are off set, and this has been going on for seasons now," insiders told the magazine. "This season, it got so out of hand they made Stana and Nathan go to couple's counseling together."
Sources told The Hollywood Reporter Fillion and Katic had "repeated clashes," but said Katic's departure was ultimately "a financial decision" for ABC. The network reportedly cut costs by letting Katic and co-star Tamala Jones go.
To the fans: thank you for your support. I love you. 💋✌🏽️— Stana Katic (@Stana_Katic) April 19, 2016
Castle airs Mondays at 10 p.m. ET and will air its Season 8 finale May 6. The series has yet to be renewed for Season 9, but is expected to return for a final, abbreviated season starring Fillion.