Siân Rebecca Berry (born 9 July 1974) is a British politician.
On 4 September 2018, she became Co-Leader of the Green Party of England and Wales alongside Jonathan Bartley. She stepped down from the party's leadership in October 2021. From 2006 to 2007, she was one of the Green Party's Principal Speakers.
She was the party's candidate in the 2008 London mayoral election, and again in the 2016 election, at which she came third.
She is a member of the London Assembly and the only Green Party councillor on Camden Council, representing Highgate.