Augusta Marsh Birney (born July 27, 1999), known professionally as Gus Birney, is an American actress. She is known for her television roles in the The Mist (2017), Dickinson (2019–2021), and Shining Vale (2022–2023).^[1] On stage, she made her Broadway debut in the 2023 revival of The Sign in Sidney Brustein's Window. She also acted in off-Broadway productions of The Rose Tattoo (2016) and Our Class (2024).

Birney is the daughter of actors Reed Birney and Constance Shulman.^[2]

Birney made her professional stage debut playing Rosa in the 2016 production of the Eugene O'Neill play The Rose Tattoo at the Williamstown Theatre Festival. She acted alongside Marisa Tomei, Christopher Abbott, and Lindsay Mendez.^[3] She made her film debut in the romantic comedy Here and Now (2018) starring Sarah Jessica Parker.^[4] She has since acted in the Charlie Kaufman Netflix psychological drama I'm Thinking of Ending Things (2020),^[5] and the Hulu coming-of-age teen comedy Plan B (2021).^[6] She took roles in the science fiction-horror series The Mist (2017), the Netflix superhero series Jessica Jones (2019), and the Netflix comedy series Insatiable (2019). She portrayed Jane Humphreys in the Apple TV+ series Dickinson (2019–2020) starring Hailee Steinfeld.^[7] From 2022 to 2023 she acted in the Apple TV+ satirical comedy horror series Shining Vale earning a Saturn Award for Best Young Actor in a Television Series nomination.

From February to March in 2023 she acted in the off-Broadway revival of Lorraine Hansberry's The Sign in Sidney Brustein's Window starring opposite Oscar Isaac and Rachel Brosnahan at the Brooklyn Academy of Music.^[8] She played a young prostitute, Gloria Parodus, the sister of Iris (Brosnahan). The production made its transfer to Broadway with Birney joining making it her Broadway debut.^[9] Birney said of the experience, "Doing Sidney Brustein was one of the most transformative experiences of my life, not only as an actor, but as a human being. Anything after feels almost like, “OK, I know I can do this. I just went through something hard, and I have taught my soul how to handle going to a really dark place."^[10] David Gordon of TheatreMania described her performance as being "excellent".^[11] The production was nominated for the Tony Award for Best Revival of a Play losing to the revival of Topdog/Underdog.^[12]

In 2024 she acted in the adaptation of the Polish playwright Tadeusz Slobodzianek's play Our Class at the Brooklyn Academy of Music.^[13] She portrays Dora, a young Jewish mother, one of 10 Polish students in the time leading up to a 1941 pogrom in the village of Jedwabne.^[14] Raven Snook of Time Out described her performance as "heartbreaking".^[15]