Obama was expected to resume campaigning on Saturday, though Gibbs was unsure where.
Obama events originally planned for Madison, Wis., and Des Moines, Iowa, on Thursday will be replaced with one in Indianapolis before he makes the long flight to Hawaii.
Gibbs said it was still being decided whether surrogates would fill in for Obama. His wife, Michelle, for instance, was not accompanying him to Hawaii.
"Senator Obama's grandmother, Madelyn Dunham, has always been one of the most important people in his life, along with his mother and his grandfather," Gibbs said. "Recently his grandmother has become ill and in the last few weeks her health has deteriorated to the point where her situation is very serious. It is for that reason that Sen. Obama has decided to change his schedule on Thursday and Friday so that he can see her and spend some time with her."
Citing the family's desire for privacy, Gibbs would not discuss the nature of Dunham's illness. It seemed likely that she was close to death, as Gibbs said that "everyone understands the decision that Sen. Obama is making."
It could be a momentous one for his bid for the White House against Republican John McCain, with Election Day just two weeks away on Nov. 4.
In a campaign ad this year, Obama described his Dunham as the daughter of a Midwest oil company clerk who "taught me values straight from the Kansas heartland" - things like "accountability and self-reliance. Love of country. Working hard without making excuses. Treating your neighbor as you'd like to be treated."
She's also the "white grandmother" he referred to in a speech on race.
Obama last visited Hawaii in August, when he spent a week on vacation after clinching the Democratic presidential nomination.