Ginsburg, 86, began radiation therapy at Memorial Sloan Kettering Cancer Center in New York City on August 5th, according to the court.
The Supreme Court said in a statement Friday that a routine blood test led to the detection of Ginsburg's tumor.
Ginsburg's health has always been an issue.
She came to the Supreme Court with a reputation as a pro-abortion feminist, once serving as counsel for the radical National Organization for Women as well as the American Civil Liberties Union, founding its Women's Rights Project, before being nominated to the High Court by President Bill Clinton in 1993. Doctors say there is no evidence the disease has spread elsewhere.
As the court's oldest member, Ginsburg has been asked questions for years about her health and how long she plans to stay on the court, even as she's attracted enthusiastic fans as the leader of the liberal wing.
President Trump has already named two conservative justices to the court, thus ensuring a five-justice conservative majority in most controversial cases. If the cancer is truly limited to the pancreas, "it could have been managed perfectly well with radiation", he said.
"There was a senator - I think it was after the pancreatic cancer - who announced with great glee that I was going to be dead within six months", she said. She attended a legal conference in Lisbon this summer, joined by Justice Sonia Sotomayor and retired Justice John Paul Stevens, who died soon after returning. She married Martin Ginsburg, and together they had a daughter and a son.