The 27-year-old posed in white wrap dress and held her arms around her belly to reveal the news, writing "Guess what" on the caption.
The actress is currently dating photographer Brick Stowell, and tagged him in the photo.
Stowell took it upon himself to share his own Instagram announcement, by posting a selfie of him and Masterson in the mirror.
Both wearing hats and shades, Masterson is in a bikini with her baby bump on full show.
The father-to-be added: "Baby season" to the caption with baby bottle emojis.
Masterson plays Chambler in the gritty drama, a former Atlanta Police Academy student who follows a Governor in his siege of the prison.
It's still a mystery as to how they will incorporate Masterson's growing baby bump with the hit drama, but time will tell.
Discussing the fate of her character in an interview, the actress admitted that she was worried Tara might be the next character to die.
She told Entertainment Weekly: "That's the nature of the show. It's interesting because if you look at Emily Kinney (Beth) or Chad Coleman (Tyreese) or Tyler James Williams (Noah) and people that have gone this season - it's done nothing but be great for them."
Season six of the show will be hitting US screens on 11 October.