Ballesteros, 54, underwent several surgeries and chemotherapy treatments for a cancerous brain tumor and last year he said his health was getting better.
But his family said on the golfer's Web site Friday his health is failing.
"The family will inform accordingly about any change in his health condition and takes this opportunity of thanking everyone for the support that both Seve and his own family have been receiving during all this time," a message said.
Ballesteros was once ranked No. 1 in the world. His many championships around the world included two Masters titles and three at the British Open.
Last May, the golfer said he was 90 percent recovered following four operations on the tumor. He also underwent several rounds of chemotherapy.
"Now I am perfect, I can do any kind of sport, running, swimming, rowing and I am also hitting golf shots," Ballesteros said then.
Family members said updates on the golfer's condition would be posted on his Web site at: www.seveballesteros.com.