Supreme Court Justice Anthony Kennedy is to retire, giving US President Donald Trump the chance to reshape the top US court for decades to come.
Though a conservative, he has proved instrumental in advancing gay rights, buttressing abortion rights and erasing political spending limits
Justice Kennedy's vote has been pivotal on key decisions including the 5-4 rulings that decided same-sex marriage and upheld Roe v Wade.
In his letter to Mr Trump, Justice Kennedy expressed "profound gratitude" for having served in the highest court.
Justice Kennedy, 82, will retire on 31 July, according to the letter.
He is the second oldest justice on the nine member court.
Republicans could confirm a conservative replacement as long as they maintain a simple majority.
Mr Trump said he was launching an immediate search for someone to replace Justice Kennedy, whom he hailed as a spectacular man with great vision and heart.
Speaking to reporters during an Oval Office meeting with Portuguese President Marcelo Rebelo de Sousa, Trump said he would draw from a list of 25 potential court candidates that his campaign had assembled during his presidential run.
Justice Kennedy was nominated by President Ronald Reagan and he began his term in 1988.
He 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.
Without Justice Kennedy on the bench, the high court could move to the right on major social issues including abortion and gay rights.
Conservative activists have long dreamed of building a firmly conservative majority on the court that would push to overturn the landmark 1973 ruling in the case Roe v. Wade that legalised abortion nationwide.
- BBC / Reuters