Doctors discovered the tumor after McCain underwent blood clot surgery last week at the Mayo Clinic in Phoenix, Arizona, the hospital said on Wednesday.
"Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot," McCain's aids confirmed in a statement.
Glioblastoma is a very aggressive form of cancer whose five-year survival rate for patients over 55 is about 4 percent, according to the American Cancer Society.
The politician and his family are considering further treatment, including a combination of chemotherapy and radiation, McCain's office said, adding that the senator was "in good spirits as he continues to recover at home with his family in Arizona."
McCain, who serves as the head of the Senate's Armed Services Committee, will consult with his medical team about returning to work in Washington. Doctors noted that he was recovering from his blood clot surgery "amazingly well" and that his underlying health was excellent.
Hanoi to Washington DC
US President Donald Trump said he and his wife Melania were sending their "thoughts and prayers" to McCain and his family. "Senator John McCain has always been a fighter," Trump said in a statement. "Get well soon."
The 80-year-old politician served as a pilot in the Vietnam War. He was shot down over Hanoi and captured in 1967, spending the next five and half years as a prisoner of war. McCain entered politics in 1982, when he successfully ran for Congress in Arizona. He is currently in his sixth term as a US senator.
Senator McCain drew international attention by running for the Republican Party against Barack Obama in the 2008 presidential election. Speculation about his ailing health was reignited last month, when McCain quizzed ex-FBI director James Comey during a widely publicized hearing on Trump's alleged ties with Russia. Appearing disoriented, McCain asked several questions on Hillary Clinton's emails and tried to tie the issues together.
The senator later joked that his apparent confusion was due to him staying up late and "watching the Diamondbacks," a baseball team from Arizona.
Daughter praises McCain's courage
The latest cancer diagnosis comes after doctors removed several malignant melanomas on McCain's skin in the 1990s and 2000s, including an invasive melanoma in 2000.
McCain's daughter Meghan McCain issued a statement after the diagnosis was made public, saying that her father remained "confident and calm."
"He is the toughest person I know. The cruelest enemy could not break him. The aggressions of political life could not bend him," she said in a statement. "So he is meeting this challenge as he has every other. Cancer may afflict him in many ways: but it will not make him surrender. Nothing ever has."
dj/bk (AFP, dpa, Reuters, AP)