Siân Thomas (born 20 September 1953) is a Welsh actress who trained at the Central School of Speech and Drama.
She has appeared on stage, on TV and in films such as Harry Potter and the Order of the Phoenix in which she played Amelia Bones. She was part of the cast of the musical Spring Awakening in London until May 2009 (when the show closed), playing the "Adult Women".
In 2002 she appeared in London's West End theatre production Up for Grabs with Madonna.
In 2012 she appeared as Atorloppe in Merlin.
She has a particularly effective voice for reading modern poetry, which is often deployed on the BBC Radio 3 programme Words and Music.
Thomas spent part of her childhood living in Canada. She is the sister of actress Sara Mair-Thomas.partner is noted British poet Tony Harrison.