The
court plays a key role in American life and is often the final word on
highly contentious laws, disputes between states and the federal
government, and final appeals to stay executions.
In recent years,
it has expanded gay marriage to all 50 states, halted President Barack
Obama's immigration orders and delayed a US plan to cut carbon emissions
while appeals went forward.
Senate Majority Leader Mitch
McConnell, a Republican, said on Wednesday that a vote on Mr Trump's
nominee to replace Justice Kennedy - who at times sided with liberal
justices and earned a reputation as the court's "swing" vote - would
take place by the autumn.
Justice Kennedy said he wanted to spend more time with his family.
Who is Anthony Kennedy?
Justice Kennedy, who was raised in California, was nominated by President Ronald Reagan and began his term in 1988.
He
voted conservative on issues of campaign finance, voting rights and gun
rights but was considered a swing vote on key rulings.
Justice Kennedy penned the Supreme Court's first major gay-rights decision in 1996, protecting LGBT Americans from discrimination.
In
2015, he authored the landmark opinion which gave LGBT citizens the
right to marry, writing: "They ask for equal dignity in the eyes of the
law and the Constitution grants them that right."
As a justice, he routinely favoured personal liberty and the limiting of federal power.
Mr Trump said Justice Kennedy had "displayed great vision" and "tremendous heart".
US Supreme Court Justice Anthony
Kennedy is to retire, giving President Donald Trump the chance to cement
a conservative majority on the top court.
The conservative has
cast a pivotal vote on many decisions including the 5-4 rulings that
decided same-sex marriage and upheld abortion rights.
In his letter to Mr Trump, Justice Kennedy expressed "profound gratitude" for having served in the highest court.
Justice Kennedy, 81, will retire on 31 July, he said in his letter.
He is the second oldest justice on the nine-member US Supreme Court.
The
court plays a key role in American life and is often the final word on
highly contentious laws, disputes between states and the federal
government, and final appeals to stay executions.
In recent years,
it has expanded gay marriage to all 50 states, halted President Barack
Obama's immigration orders and delayed a US plan to cut carbon emissions
while appeals went forward.
Senate Majority Leader Mitch
McConnell, a Republican, said on Wednesday that a vote on Mr Trump's
nominee to replace Justice Kennedy - who at times sided with liberal
justices and earned a reputation as the court's "swing" vote - would
take place by the autumn.
Justice Kennedy said he wanted to spend more time with his family.
Who is Anthony Kennedy?
Justice Kennedy, who was raised in California, was nominated by President Ronald Reagan and began his term in 1988.
He
voted conservative on issues of campaign finance, voting rights and gun
rights but was considered a swing vote on key rulings.
Justice Kennedy penned the Supreme Court's first major gay-rights decision in 1996, protecting LGBT Americans from discrimination.
In
2015, he authored the landmark opinion which gave LGBT citizens the
right to marry, writing: "They ask for equal dignity in the eyes of the
law and the Constitution grants them that right."
As a justice, he routinely favoured personal liberty and the limiting of federal power.
Mr Trump said Justice Kennedy had "displayed great vision" and "tremendous heart".
Anthony Zurcher, BBC News, Washington
Without
Anthony Kennedy, the political centre on the Supreme Court will be
firmly on the right. Whoever Donald Trump nominates - and he's promised
to draw from the same list of candidates from which he picked Neil
Gorsuch - will be a person with solid conservative bona fides.
Democrats
and liberal activists will howl, rage and do all they can to slow the
process, but the timing of Mr Kennedy's retirement makes what happens
next all but inevitable. The president will surely offer his choice
quickly, and if Republican senators stick together they have the votes to confirm before November's mid-terms congressional elections, let
alone when new senators are sworn in next January.
And even if the
court vacancy becomes a campaign issue, the open seat in 2016 proved
that court vacancies are a much stronger motivating consideration for
Republicans - particularly evangelical voters - than they are for
Democrats.
File this as yet another entry in the "elections have consequences" scrapbook.
If
Hillary Clinton had prevailed in 2016, Democrats would be contemplating
a 6-3 liberal Supreme Court majority and an opportunity to reshape the
legal landscape for a generation. Instead, on subjects like abortion law
and gay rights, the pendulum is swinging the other direction.
