Population Matters, formerly known as the Optimum Population Trust, is a UK-based charity that works at the intersection of population, environmental sustainability, and human rights, including women's empowerment and leadership, sexual and reproductive health and rights (SRHR), and international development. The group promotes human rights and ethical, choice-based solutions through research, campaigning and awareness-raising.

Population Matters was launched as the Optimum Population Trust following a meeting on 24 July 1991 by the late David Willey and others concerned about population numbers and sustainability. They were impelled to act by the failure of United Kingdom governments to respond to population growth and threats to sustainability.

The Optimum Population Trust prepared analyses and lobbied on issues affected by population growth. It was granted charitable status on 9 May 2006. Population Matters was adopted as its new name in 2011.

Population Matters highlights how rapid human population growth has fueled the destruction of nature and natural resource depletion. The charity promotes positive, voluntary measures to achieve a sustainable human population size that enables everyone to have a decent quality of life while safeguarding our natural environment.^{[citation needed]}

Population Matters' vision is of a world in which our human population lives fairly and sustainably with nature and each other.

Population Matters' mission is to address the negative consequences of ever more people using ever more of the planet’s resources and to inspire and engage with others to find, share and promote ways to make our vision a reality as quickly as possible.

Population Matters promotes various solutions to slow and ultimately reverse population growth:

In addition, recognising the disproportionately large environmental footprint of wealthy nations, the charity calls for reducing consumption in high-income countries.