'Shameless' star Cameron Monaghan leaving Showtime series

'Shameless' star Cameron Monaghan announced on Monday he is leaving the Showtime series.  (Andrew Chin/Getty Images)

One month after Emmy Rossum announced her "Shameless" exit, another star is following suit.

Cameron Monaghan, who has played Ian Gallagher since "Shameless" debuted in 2011, is departing the Showtime series midway through its ninth season. The 25-year-old actor, whose final episode will air Sunday, Oct. 14, made the official announcement on Instagram on Monday, one week after a promo seemingly teased his departure.

"All good things come to an end. An old cliche, but one that rings true with a sincerity and clarity especially in moments like these. Everything ends," Monaghan wrote in a lengthy post. "The next episode will be my last."

William H. Macy Admits He 'Cried' at Emmy Rossum's Last 'Shameless' Table Read (Exclusive)

The actor, who also stars on Fox's Batman prequel series, "Gotham," revealed that his decision to leave has been long in the works.

"I have known since last year, but I didn’t want to give it away too early as I wanted this season to be a surprise for the audience, allowing them to experience Ian’s unsure journey with his character," Monaghan explained. "This role has been a joy to inhabit, a wild and special ride, and I’d like to thank #Shameless as well as you, the viewers, for being there with him."

William H. Macy Reacts to Emmy Rossum's 'Shameless' Exit

"Goodbye, Ian Clayton Gallagher. We’ll meet again?" he concluded his post, leaving the door open for a potential return.

Monaghan's departure comes on the heels of Rossum's surprise announcement that she would be leaving "Shameless" following a nine-season run playing eldest Gallagher sibling, Fiona.

"The opportunity to play Fiona has been a gift," Rossum wrote in a letter posted in August. "There are few characters -- female or otherwise -- as layered and dynamic. She is a mother lion, fierce, flawed and sexually liberated. She is injured, vulnerable, but will never give up. She is living in an economic depression, but refuses to be depressed. She is resourceful. She is loyal. She is brave.

"I knew it the second I read the pilot script, this was different, this was special. I tirelessly prepped the audition with my coach Terry Knickerbocker. I walked to the appointment in the rain so I looked disheveled. During my third audition, when I got the part IN the room, I literally jumped up and down screaming in joyous relief and disbelief. Quite simply, the last eight years have been the best of my life.

Emmy Rossum Says She's Leaving 'Shameless' After 9 Seasons

"I know you will continue on without me, for now. There is much more Gallagher story to be told," she concluded. ("Shameless" has not been officially picked up for a 10th season.) "I will always be rooting for my family. Try not to think of me as gone, just think of me as moving down the block."

William H. Macy revealed to ET that he was emotional during Rossum's final table read for the show.

"I'm the one that blew it. We did a table read for her last episode, I cried right in the middle of the table read in front of everyone," Macy told ET over the weekend.

"It was so bad and my voice turned into some weird voice I'd never heard as I was trying to muscle through it and then everybody else started crying, so at least I didn't look like a complete jerk."

"Shameless" airs Sundays at 9 p.m. ET/PT on Showtime.