"It's a wonderful day in Maine — in fact, nice enough for a parachute jump," the nation's 41st president announced on Twitter.
Bush intends to jump from a helicopter with a retired member of the Golden Knights, the Army's parachute team, near his summer home on the Maine coast, spokesman Jim McGrath said.
The announcement was kept secret until the last minute, partly to give Bush himself the option of bagging it. The forecast Thursday called for clouds and scattered showers across southern Maine.
McGrath said Bush likes both a surprise and an adrenaline rush.
"It's vintage George Bush," he said. "It's that passion for life. It's wanting to set a goal, wanting to achieve it. I'm sure part of it is sending a message to others that even in your retirement years you can still find challenges."
The first time Bush jumped from an airplane was when his plane was shot down in World War II over the Pacific. Later, he decided to jump from a plane of his own accord and marked his 75th, 80th and 85th birthdays by skydiving.
He said on his 85th birthday that he'd like to do it again on his 90th. Thursday's jump would be his eighth.
Other birthday festivities included a private dinner with more than 200 relatives and friends, including some from his White House days: press secretary Marlin Fitzwater, White House counsel Boyden Gray and political director Ron Kaufman, McGrath said. His children, including former President George W. Bush and former Florida Gov. Jeb Bush, will be there, McGrath said.
Kennebunkport is a special place for the president. As a boy, he visited the family home at Walker's Point every summer, except during World War II. The retreat was later dubbed his "summer White House."
During his presidential years, Bush was known for jogging, tennis and fast-paced golf but now uses a wheelchair or scooter because of a form of parkinsonism that has robbed him of use of his legs.
"He's lost his mobility, but he hasn't lost his heart. He's still the genuine person that we've come to cherish," said Ken Raynor, a friend and pro at the Cape Arundel Golf Club.
While his activities are now limited, Bush still fulfills his need for speed on his boat, Fidelity. "He's always loved going fast. He loves the speed. He loves the adrenaline," McGrath said.
Jon Meacham, a Pulitzer Prize-winning author and history who is writing a book about Bush, said the former president is used to being in motion, so it isn't easy for him to slow down.
The president feels lucky nonetheless, Meacham said.
"He had a remarkable great run of good health and good family and good friends," he said. "So I know his chief view of life at 90 is one of immense gratitude. He's very grateful for his parents, he's grateful for Barbara, he's grateful for his kids. He knows he's one of the luckiest guys who ever lived, really."