Masterson revealed the happy news to Instagram followers by sharing a picture of herself cradling her baby bump, along with the caption: "Guess what", reported Digital Spy.
This will be the first child for the New York-born actress and her boyfriend, photographer Brick Stowell.
It isn't clear yet if Masterson's pregnancy will be written into "The Walking Dead", on which she plays Tara Chambler.
Alanna is a member of the Masterson acting family, and is the younger half-sister to "That '70s Show" star Danny Masterson and Christopher Masterson ('Malcolm in the Middle').