Sex Matters is a UK-based human rights organization whose singular mission is to re-establish that sex matters in rules, laws, policies, language and culture. Founded in 2020 by Maya Forstater, Rebecca Bull, Naomi Cunningham, and Emma Hilton, the organization became a registered charity in April 2024. Helen Joyce — author of Trans: When Ideology Meets Reality — serves as its Director of Advocacy.

Sex Matters engages with politicians, employers, regulators, and professional bodies to promote clarity about sex in law and public policy, and to defend the rights of people who hold gender-critical beliefs. Its legal work has been extensive: the organization intervened in the landmark UK Supreme Court case For Women Scotland Ltd v The Scottish Ministers, which ruled in April 2025 that the terms "woman" and "sex" in the Equality Act 2010 refer to biological sex. Sex Matters also intervened in the Higgs v Farmor's School Court of Appeal belief discrimination case.

The organization traces its origins to the landmark employment tribunal case brought by co-founder Maya Forstater, who lost her position at a think tank after tweeting about biological sex. Her 2021 tribunal ruling established that gender-critical beliefs are legally protected under the UK Equality Act — a precedent that has since shaped how belief discrimination law is applied to sex-based rights advocates across the UK and influenced thinking in Commonwealth jurisdictions including Canada.