McCain, 81, a Vietnam War hero and 2008 Republican presidential nominee, has been an Arizona senator since 1987. He was diagnosed with brain cancer and spent most of his time undergoing treatment in Arizona.
Family members gathered in Arizona and those close to the family have said that McCain's death is imminent, The New York Times reported Friday.
"I love my husband with all of my heart. God bless everyone who has cared for my husband along this journey," wife Cindy McCain said.
"My family is deeply appreciative of all the love and generosity you have shown us during this past year," daughter Meghan McCain said. "Thank you for all your continued support and prayers. We could not have made it this far without you -- you've given us strength to carry on."
McCain's family released a statement indicating his decision to end treatment.
"Last summer, Senator John McCain shared with Americans the news our family already knew: he had been diagnosed with an aggressive glioblastoma, and the prognosis was serious. In the year since, John has surpassed expectations for his survival," it said.
"But the progress of disease and the inexorable advance of age render their verdict. With his usual strength of will, he has now chosen to discontinue medical treatment. Our family is immensely grateful for the support and kindness of all his caregivers over the last year, and for the continuing outpouring of concern and affection from John's many friends and associates, and the many thousands of people who are keeping him in their prayers. God bless and thank you all."
Friday's news drew an outpouring of sentiment from colleagues on Capitol Hill and in the U.S. Navy.
"Senator John McCain has always been a fighter, a patriot, and a dedicated father and husband. I want to extend my thoughts and prayers to him and his family, and I wish him nothing but the best," Rep. Elijah Cummings, D-Md., said. "I know his courageous spirit will lead him."
"Very sad to hear this morning's update from the family of our dear friend Sen. John McCain. We are so fortunate to call him our friend and colleague," Senate Majority Leader Mitch McConnell said. "John, Cindy, and the entire McCain family are in our prayers at this incredibly difficult hour."
"They just don't make 'em like John McCain anymore," Virginia Sen. Tim Kaine said. "For 60 years, he has served our country. As a man in uniform. As a maverick leader always seeking common ground. As a committee chairman dedicating his work to serving our troops and veterans. We love you, John. We're with you."
"Senator John McCain embodies Honor, Courage and Commitment. He's an inspiration to all, especially to those of us in the Naval Service," Adm. John Richardson said. "On behalf of the U.S. Navy, we will keep Senator McCain and the McCain family in our thoughts and prayers."