The senator is expected to be treated with combinations of various forms of radiation and chemotherapy, however experts told AP the tumour would most likely be fatal.
Decisions regarding specific treatments would be made "after further testing and analysis," doctors said.
The diagnosis comes six months after the senator had surgery to clear a blockage in a major neck artery, a common procedure to prevent a stroke.
George Bush, the US president, released a statement on Tuesday praising Kennedy as a man of "tremendous courage, remarkable strength, and powerful spirit" and said he and his wife Laura's thoughts "are with Senator Kennedy and his family during this difficult period."
Al Jazeera's Rosalind Jordan said Kennedy is known as a "liberal lion...someone who has always stood up for the interests of working people and the poor."
First elected to the US senate in 1962, Kennedy is the second longest-serving senator after Robert Byrd of West Virginia.
He ran for the Democratic presidential nomination in 1980, but lost the race to incumbent president, Jimmy Carter.
The only surviving brother of John F Kennedy, who was assassinated on November 22, 1963 in Dallas, Texas, Kennedy is a well-known supporter of liberal causes and endorsed Barack Obama, the Democratic presidential hopeful.
His brother Robert Kennedy was also assassinated in 1968 shortly after he announced his bid for the US presidency and his eldest brother, Joseph Patrick Kennedy, was killed in World War II.
In 1969 Kennedy accidentally drove his car off a bridge as he headed to Massachusetts' Chappaquiddick Island and a young female campaign worker he was with drowned.
Kennedy, who did not call authorities until the next day, pleaded
guilty to leaving the scene of an accident and received a suspended two-month jail sentence.