Allen, who has been on the show since its first season, couldn't help but get emotional bidding farewell to his co-stars -- and on-screen love Troian Bellisario in particular.
"I absolutely love you Troian Bellisario," Allen captioned an Instagram snap of the actress playing guitar. "And as our characters drift off into space and away from our control, I remain grateful I could inhabit their existence with you right there to ground and protect me. I will miss these subtle moments and quick, brief glimpses of you and this whole surreal experience."
Bellisario responded to Allen's heartfelt message with one of her own on Tuesday.
"There are not words that I can write (here) that could express the gratitude I have for walking next to this man for 7 years," she wrote. "@keeoone you know how I feel about you. You know what this part of my life means to me, you know all of the darkness and the light that we have shared over these years, the insane pants pissing laughter and now the heaving and hiccuping sobs that will live in my heart forever. Even if Spencer and Toby only exist in Rosewood you will always have me. Love you friend. Thank you."
ET caught up with Allen's co-star, Lucy Hale, on Monday, where she assured fans that while they're sad to say goodbye, the PLL season finale is full of "happy moments."
"There will be happy moments. It is in the nature of our show," she said. "Things can't go perfectly well. But there are happy moments. You will smile, you will cry."
Season seven of Pretty Little Liars returns to Freeform in April 2017, and as announced on Wednesday, the series finale will be followed by an hourlong after-show with stars Bellisario, Hale, Ashley Benson, Shay Mitchell, Sasha Pieterse and executive producer I. Marlene King.
See more on what Hale had to say about the big finale in the video below.