Dame Siobhain Ann McDonagh DBE (born 20 February 1960) is a British Labour Party politician who has been the Member of Parliament (MP) for Mitcham and Morden since 1997.

She served as an Assistant Whip in the Labour Government, but was dismissed following comments regarding a leadership contest to replace prime minister Gordon Brown.

She also served as a councillor on London Borough of Merton for Colliers Wood ward between 1982 and 1998, chairing the Housing Committee between 1990 and 1995, being instrumental in the rebuilding of Phipps Bridge Estate.

[8] After the 2001 general election, Tony Blair offered McDonagh the position of Parliamentary Undersecretary of State at the Department for Communities and Local Government.

[22] McDonagh abstained from a vote on a motion withdrawing the UK's support for Saudi Arabia's military campaign in Yemen.

[24][25][26] In September 2018, McDonagh offered her support to Labour MP Chris Leslie when he faced a confidence motion from his CLP, a vote he subsequently lost.

[27] In March 2019, McDonagh was criticised by some left-wing members of the party after she appeared to agree with a statement put forward by John Humphrys on BBC Radio 4's Today programme.

[2] In March 2023 McDonagh took a £1.2 million interest-free loan from Labour donor Lord Alli, to buy a house in south-west London with downstairs bedroom and bathroom suited for her terminally ill sister.

[40] She was a patron of Leap Forward Employment – a now defunct community interest company that found work for adults with mental health issues.

[41] McDonagh was appointed Dame Commander of the Order of the British Empire (DBE) in the 2024 New Year Honours for parliamentary and political service.