Lia Williams (born 26 November 1964) is an English actress and director, on stage, in film, and on television. She has had television roles in The Crown (playing Wallis Simpson), in May 33rd (2004) for which she was nominated for a BAFTA, and in The Missing (2016), Kiri (2016), His Dark Materials (2019–2022) and The Capture (2019–2026).

Williams is a three-time Laurence Olivier Award nominee, for her work in The Revengers' Comedies (1992), Skylight (1997), and Oresteia (2015).

Williams was born in Birkenhead. She graduated from the London Studio Centre. Her first job in 1984 was understudying in the play Daisy Pulls It Off in the West End. She then took over a main role, and was talent-spotted by Alan Ayckbourn.

Williams's breakthrough performance came in 1991 when she appeared in The Revengers' Comedies, for which she won the Critics' Circle Theatre Award for Most Promising Newcomer, and was nominated for the Laurence Olivier Award for Best Comedy Performance in 1992.

In 1992, she played the role of Carol in the original production of David Mamet's Oleanna, at the Theatre Royal, Bath, transferring to the Royal Court Theatre in 1993.

In 1995, Williams appeared opposite Michael Gambon in David Hare's Skylight at the National Theatre. She received a nomination for the 1996 Laurence Olivier Award for Best Actress. Williams reprised her role in the 1997 Broadway production, and was nominated for the Tony Award for Best Actress.

In 2001, Williams appeared again in the West End and on Broadway, playing Ruth in Harold Pinter's The Homecoming. Her long-standing collaboration with Harold Pinter included roles in The Collection, Celebration, The Room, The Lover, The Hothouse and Old Times.

Other leading theatre performances include Rosalind in As You Like It for the RSC, Alan Ayckbourn's Absurd Person Singular, Henryk Ibsen's The Lady from the Sea, and Pinter's Old Times, in which she alternated the roles of Anna and Kate with Kristin Scott Thomas.