Martin-Green has been cast as the mysterious new character Tamara, confirms Entertainment Weekly.
Tamara - billed as "a charming and likeable woman who's secretly highly ambitious and a fearless leader" - is expected to be a recurring role this season and could also appear in season three.
The actress played Sasha in the Walking Dead mid-season finale and will continue in the role when the show returns to AMC in February.
Martin-Green's other TV roles have included The Good Wife and Army Wives.
Watch a trailer for the latest US episode of Once Upon a Time 'The Cricket Game' below:
Once Upon a Time's second season airs in the UK on Channel 5 in the spring.