Mrs. Bush, 92, has been hospitalized several times in Houston, Texas, over the past year to be treated for chronic pulmonary disease and congestive heart failure, her family has said.
“Following a recent series of hospitalizations, and after consulting her family and doctors, Mrs. Bush, now age 92, has decided not to seek additional medical treatment, and instead will focus on comfort care,” reads a statement released by her husband's office.
"It will not surprise those who know her that Barbara Bush has been a rock in the face of her failing health, worrying not for herself -- thanks to her abiding faith, but for others," the statement reads. "She is surrounded by a family she adores and appreciates the many kind messages and especially the prayers she is receiving."
In January, Mrs. Bush and her husband were hospitalized at the same time. She was being treated for bronchitis and the nation's 41st president was being treated for pneumonia.
Barbara Bush served as the country's first lady from 1989 to 1993. She is the mother of George W. Bush, the 43rd president of the United States.
Mr. and Mrs. Bush celebrated their 73rd wedding anniversary on Jan. 6.
In an alumni magazine from Smith College, where she attended, Mrs. Bush says the marriage has endured because of the couple's love for each other.
"I am still old and still in love with the man I married 72 years ago," she said at the time in the "Smith Alumnae Quarterly."
"George Bush has given me the world," she also said in the magazine, which was published earlier this month. "He is the best -- thoughtful and loving."
Heartfelt support for the former first lady came pouring in from all over the country.
"The President's and First Lady's prayers are with all of the Bush Family during this time," White House Press Secretary Sarah Huckabee Sanders said in a statement.
"As always, Barbara Bush is a comfort to her friends & family teaching us all how to live full of faith, love & humor. Praying that she is comfortable & send love to President Bush & their family," Pelosi Tweeted.
"Prayers going up for a woman of great faith, great strength, and an unwavering love of country," Haley tweeted. "Our country is better because of former First Lady Barbara Bush."
"I will never forget how kind she was to me on every occasion we met, and how fondly the White House staff always spoke of her," Clinton wrote.
During her tenure as the nation's first lady, Barbara Bush, the mother of six children, was a champion for global literacy and continued the work after her and her husband left the White House. In 1989, she formed the Barbara Bush Foundation for Family Literacy, which encourages parents to read to their children.
"Literacy fits in with so many other things," she once told the Chicago Tribune. "If more people could read, fewer people would have AIDS. There would be less homelessness. I'm absolutely convinced of that."
Mrs. Bush has had a long history of health problems, dating back to her time as the first lady. In 1989, she revealed that she suffered from Graves' Disease, an autoimmune disorder that affects the thyroid. She later began taking medications for the disease and received radiation as part of her treatment.
After experiencing abdominal pains in November 2008, Mrs. Bush was hospitalized at Methodist Hospital in Houston in November of 2008. She underwent laparoscopic surgery to close a hole in her small intestine caused by an ulcer.
Four months later, she underwent precautionary open heart surgery at the same hospital to replace her aortic valve. Hospital officials reported that the procedure was a success.
During her four years as the first lady, and eight years before that as the wife of the vice president in the Reagan administration, Mrs. Bush embodied what many believed to be the traditionally dutiful political wife: silent on most issues, but enthusiastically supportive of her husband and family.
In her 1994 book -- "Barbara Bush: A Memoir" -- she revealed her pro-choice stance on abortion, a position that ran contrary to her husband's.
But speaking to Barbara Walters on ABC News' "20/20" upon publication of the book, Barbara Bush indicated she was sincere about the emphasis the book placed on her family and friends. In the book, she reproduced an open letter to her children containing her advice on life.
"Value your friends," she wrote. "They are your most valuable asset.
"Love your children," she added. "You are the best children any two people ever had. I know you will be as lucky. Your kids are great. Dad and I love them more than life itself."
Julie Percha and M.L. Nestel contributed to this report.