Katherine Ruth Gallagher (/'gæləhər/ GAL-ə-her; born 18 March 1970) is an Australian politician who has been serving as the Minister for Finance, Minister for Women, Minister for the Public Service and Vice-President of the Executive Council in the Albanese Government since 2022 and since 2025 as Minister for Government Services. She also formerly served as the 6th Chief Minister of the Australian Capital Territory from 2011 to 2014. She has been a Senator for the Australian Capital Territory since the 2019 federal election, as a member of the Australian Labor Party (ALP) having previously served in the Senate from 2015 to 2018.

Gallagher grew up in Canberra and was a social worker and union organiser with the Community and Public Sector Union (CPSU) before entering politics. She was elected to the Australian Capital Territory Legislative Assembly in 2001, representing the electorate of Molonglo. She was made a minister under Jon Stanhope in 2002, and appointed Deputy Chief Minister in 2006. Gallagher became Chief Minister in 2011 after Stanhope's retirement, and led her party to a fourth consecutive term at the 2012 general election. She resigned in 2014 to seek preselection to the Senate.

In March 2015, Gallagher was appointed to fill the casual vacancy caused by the retirement of Senator Kate Lundy. She was appointed to Bill Shorten's shadow ministry later in the year, and elected to the Senate in her own right at the 2016 federal election. She was subsequently elected Manager of Opposition Business in the Senate. In December 2017, during the parliamentary eligibility crisis, Gallagher was referred to the High Court. The court ruled in May 2018 that she was disqualified from sitting in the Senate for failing to renounce her British citizenship before nomination for election in 2016. She returned to her previous Senate seat at the 2019 federal election.

Katherine Ruth Gallagher was born on 18 March 1970 in Waramanga, a suburb in the Weston Creek district of Canberra, to Betsy and Charles Gallagher. Her father was born in Stoke-on-Trent, England, and her mother in Guayaquil, Ecuador. Both were British citizens who later became Australian citizens after their arrival from England via New Zealand in 1969. She has an elder sister, Clare, along with two younger brothers who were adopted, Richard and Matthew. Her father died in 1995 of lung cancer and her mother in 2005 with peritoneal cancer .

Educated at Duffy Primary School, Melrose High School and Canberra College (previously known as Phillip College), Gallagher completed her studies by obtaining a Bachelor of Arts in Political Science and Sociology at the Australian National University in 1990.

She also had training as a cellist, and played with the Canberra Youth Orchestra.

Gallagher was initially employed as a social worker, assisting with a community life skills project and working with children with disabilities. From 1994 to 1997, she worked as an advocate for People First ACT, a support and advocacy organisation for the intellectually disabled.

On 30 January 1997, Gallagher's fiancé, Brett Seaman, was killed in a cycling accident in Merimbula . At the time, Gallagher was 13 weeks pregnant with her first daughter. An 86-year-old female pensioner narrowly escaped a jail term for dangerous and irresponsible driving for the crash. The union movement assisted Gallagher with the funeral and court case that followed the accident.