The event is to take place Feb. 17 at the JW Marriott Los Angeles L.A. LIVE.
"When I first accepted the honor of hosting the Writers Guild Awards, I was confused and actually thought I was receiving one," Fillion said in a statement Monday. "Since I play a writer on TV, I felt perhaps someone was under the impression I deserved an award and I wasn't about to correct them. However, now I'm in the perfect position to present myself with whichever award I choose. Who's going to know? At the very least, I can network with the most talented writers in the business in preparation for my next round of unemployment. It's a win/win."
"We think Nathan's an extraordinary choice to host the 2013 Writers Guild Awards," said the ceremony's executive producer Cort Casady. "Not only does he play a writer brilliantly on 'Castle,' but also, in addition to acting, he sings, dances, is a popular voice talent, and has a great gift for comedy. And with over 1.5 million Twitter followers, Nathan brings a smart, enthusiastic audience to our celebration of writing."
Fillion is also known for his work on the TV shows "One Life to Live," "Buffy the Vampire Slayer" and "Firefly."