Breyer, 83, is the oldest member serving on the nation's highest court after being nominated during former President Bill Clinton's administration in 1994.
Breyer's reported decision allows President Joe Biden to nominate a successor to serve both in the short and long term, maintaining the 6-3 split between conservative and liberal Supreme Court justices.
Numerous liberal activists urged Breyer to retire while Democrats held both the White House and Senate, which would allow the Biden administration to appoint a Democratic successor.
In 2020, Justice Ruth Bader Ginsburg died following a long history of health problems and was replaced by successor Amy Coney Barrett, who was appointed by former President Donald Trump quickly after Ginsburg's death and months before losing the 2020 presidential election, which also marked a shift in the Senate for the Democrats.
The progressive group Demand Justice hired a truck to circle the Supreme Court's neighborhood last year with a sign reading: "Breyer Retire. It's time for a Black woman Supreme Court justice," which President Biden has vowed to oblige to.
Several likely successors include federal Judge Ketanji Brown Jackson -- who had previously served as a law clerk under Breyer -- and Leondra Kruger, a California Supreme Court justice.
Jackson was appointed for a seat on the U.S. Circuit Court of Appeals for the District of Columbia last year and approved by a 53-44 vote, which included all 50 Democrats and three Republicans, in June, succeeding Merrick Garland, who resigned to accept the attorney general position within the Biden administration.