Citizens Advice (previously Citizens Advice Bureau and also known as Cyngor ar Bopeth in Welsh) is a British independent organisation specialising in confidential information and advice to assist people with legal, debt, consumer, housing and other problems in the United Kingdom.

The twin aims of the Citizens Advice service are "to provide the advice people need for the problems they face" and secondly "to improve the policies and principles that affect people's lives". This research and campaigns agenda also known as "social policy" is more preventative in nature and designed to stop problems arising in the first place.

Citizens Advice organisations emerged in the 1930s linked to the emergence of a fledgling social welfare service and the outbreak of World War II. Public funding for the organisation was cut following the war but restored during the 1960s and a government grant in 1973 allowed the National Association of Citizens Advice Bureaux (NACAB) to expand the charity. Citizens Advice has grown to be the largest independent advice provider in the United Kingdom. There are also a number of Citizens Advice organisations that base themselves on the United Kingdom advice charity mainly in parts of the Commonwealth including Western Australia, New Zealand, and Gibraltar.

In 2013 the Citizens Advice Adviceguide website was visited by one third of United Kingdom's online population and Citizens Advice's own research shows that four in ten of the British population contact Citizens Advice at some point during their lives. In 2014 Citizens Advice celebrated its 75th anniversary and in 2015 the charity was named Charity of the Year at the 2015 Charity Awards. During the ten year leadership of the former Chief Executive Gillian Guy Citizens Advice expanded its remit taking on the contract for the Witness Service and the face-to-face advice element of Pension Wise.

The origins of the modern Citizens Advice service can be traced back to the Betterton Report on Public Assistance from 1924. This report recommended that advice centres should be set up to offer members of the public advice to help them with their problems. During the 1930s, as preparations and plans were drawn up for the possibility of war, the role that the voluntary sector should have was determined. The National Council for Social Service (NCSS) called a meeting in 1938 in which plans to establish "Citizens Aid Bureaux" were devised in the event of war. The first 200 bureaux opened on 4 September 1939, four days after World War II started. Many of these initial bureaux were run by "people of standing" in the community. In The Story of The Citizens' Advice Bureaux Brasnett states that the typical bureau would include "a committee chairman the editor of a respected county paper, as treasurer a local bank manager; and among the members the manager of the employment exchange, an estate agent who was able to lend part of a house for offices". Brasnett also describes a range of groups running provincial bureau including Toc H, Rotary Clubs and Soroptomist Clubs. Brasnett states in The Story of the Citizens' Advice Bureau that these first bureau were mainly offshoots of established organisations in London and other large provincial cities. These organisations included the Charity Organisation Society (now the Family Action) and the London Council of Social Service, the Liverpool Personal Service Society, the City of Glasgow Society of Social Service, the Birmingham Citizens' Society (now the Birmingham Council of Social Service). By 1942, there were 1,074 bureaux in a wide range of improvised offices such as cafes, church halls, private homes and air-raid shelters. Sheffield set up in the cloisters of Sheffield Cathedral after its premises were bombed during World War II and another bureau worked in Chislehurst Caves. Mobile offices also became important in ensuring that people could access advice. Many of the issues dealt with during that time were directly related to the war. These included the tracing of missing servicemen or prisoners of war, evacuations, pensions and other allowances. The Independent describes Citizens Advice at this time as "clearing houses for family and personal problems that abound from war conditions", with common issues being lost ration books and debt issues as men went off to fight in World War II.

Many war time bureaux closed at the end of the war, although it was apparent that there was still a need for the services that had been established. A particular problem was the chronic housing shortage in the years immediately following the end of the war. In the 1950s, the funding was cut and by 1960 there were only 415 bureaux. The Citizens Advice service continued due to charitable support from groups such as the Nuffield Foundation, Carnegie Trust and the Joseph Rowntree Foundation. In 1972, The Citizens Advice service became independent. Before then, the national organisation was part of NCSS (National Council of Social Services) and most bureaux were run by the local CVS (Council for Voluntary Service). In 1973, the government funded NACAB, the National Association of Citizens Advice Bureaux, to enlarge the network. The 1984 afternoon television drama series Miracles Take Longer depicted the type of cases that a 1980s branch would have to deal with. David Harker became CEO in 1997. He led Citizens Advice until 2010 and during that time oversaw a massive IT overhaul.

Since 2003, the operating name of the National Association of Citizens Advice Bureaux changed to Citizens Advice and Citizens Advice Cymru or Cyngor ar Bopeth in Wales. In the same year Citizens Advice became the first advice sector organisation to begin to audit the quality of their advice. In 2008/9, there were 416 member bureaux offering advice from over 3,300 locations in England and Wales and a further 22 bureaux in Northern Ireland all of which are independent charities. Despite the large number of volunteers working for the organisation, level of demand for the service often far outstrips resources. Citizens Advice has recently begun looking at ways to reach all members of the community through new mediums such as email advice and digital TV. Another initiative has been allowing university students to train as advisers to gain credits toward their degree. This was pioneered by a partnership between the University of Portsmouth and Portsmouth Citizens Advice Bureau and is also now available at Birmingham City University, University of Reading, University of Northampton, Glasgow Caledonian University, and University of Glasgow. Training as a Citizens Advice adviser can earn a person up to six months off a training contract if training as a solicitor.

Gillian Guy became Chief Executive in 2010 and under her leadership a modernisation process took place which involved a rebrand of the Citizens Advice service and the introduction of new advice methods including webchat. She was succeeded by Clare Moriarty in April 2021.