John Thomas Alderdice, Baron Alderdice (28 March 1955— ) is a Northern Ireland politician. He was Speaker of the Northern Ireland Assembly 1998-2004, leader of the Alliance Party of Northern Ireland and sits in the House of Lords as a Liberal Democrat.

Alderdice was born to Reverend David Alderdice and Annie Margaret Helena, née Shields. He was educated at Ballymena Academy and Queen's University, Belfast. In 1977, he married Joan Hill, with whom he had two sons and one daughter. He has worked as a consultant psychiatrist in psychotherapy since 1988. He has also lectured at Queen's University's Faculty of Medicine.

The Alliance Party was formed in 1970 as an alternative to sectarian politics. Alderdice became its leader ahead of the 1987 general election and contested Belfast East for the party. He received 32.0% of the vote, the highest percentage every achieved in an individual seat in a Westminster election, while Alliance were firmly established on the political map. In 1988, in Alliance's keynote post-Anglo Irish Agreement document, "Governing with Consent", Alderdice called for a consociational devolved power-sharing government. Throughout the late 1980s and early 1990s, Alliance's vote stabilised at between 7% and 10%.

Alderdice once again contested Belfast East in the 1992 general election. He led the Alliance Delegation to the Forum for Peace and Reconciliation at Dublin Castle and the Northern Ireland Multiparty Talks, and was a member of the Northern Ireland Forum . He was elected to the Northern Ireland Assembly for Belfast East in 1998 and became the Assembly's first speaker, serving until 2004, although the Assembly had been suspended for much of that time. He resigned as party leader to take the position of Speaker. He has been a member of the Independent Monitoring Commission since 2003.

Alderdice was created a life peer in 1996 as Baron Alderdice of Knock in the City of Belfast. He sits in the House of Lords as a Liberal Democrat.

Alderdice has been awarded several honours: the John F Kennedy Profiles in Courage Award in 1998; the W. Averell Harriman Democracy Award in 1998; the Silver Medal of Congress of Peru in 1999 and 2004; the Medal of Honour, College of Medicine of Peru in 1999; and the Freedom of the City of Baltimore in 1991.