And even when he was eventually discovered by local hunters on the Pacific island, on 24 January 1972, the 57-year-old former soldier still clung to the notion that his life was in danger.
"He really panicked," says Omi Hatashin, Yokoi's nephew.
Startled by the sight of other humans after so many years on his own, Yokoi tried to grab one of the hunter's rifles, but weakened by years of poor diet, he was no match for the local men.
- Born in 1915 and conscripted in 1941 to serve in Manchuria, before being sent to Guam in 1944
- On his return to Japan he expressed embarrassment at having returned alive, rather than dying in the service of the emperor
- Japan had changed utterly during his three-decade absence - some found his stoicism and loyalty inspiring, others found it absurd
- He married in 1972, within months of his return and died in 1997, aged 82
- He longed to meet Emperor Hirohito - in the end he was granted an audience with Emperor Akihito in 1991
Using Yokoi's own memoirs, published in Japanese two years after his discovery, as well as the testimony of those who found him that day, Hatashin spent years piecing together his uncle's dramatic story.
His book, Private Yokoi's War and Life on Guam, 1944-1972, was published in English in 2009.
"I am very proud of him. He was a shy and quiet person, but with a great presence," he says.
Underground shelter Yokoi's long ordeal began in July 1944 when US forces stormed Guam as part of their offensive against the Japanese in the Pacific.
The fighting was fierce, casualties were high on both sides, but once the Japanese command was disrupted, soldiers such as Yokoi and others in his platoon were left to fend for themselves.
"From the outset they took enormous care not to be detected, erasing their footprints as they moved through the undergrowth," Hatashin said.
In the early years the Japanese soldiers, soon reduced to a few dozen in number, caught and killed local cattle to feed off.
The last holdouts
- Second Lieutenant Hiroo Onoda led a guerrilla task force on the Philippine island of Lubang for many years after the end of the war. He doggedly refused to lay down his arms until formally ordered to surrender. Repatriated March 1974.
- Private Teruo Nakamura, a conscript from Taiwan, was found growing crops alone on the Indonesian island of Morotai in December 1974. He was repatriated to Taiwan where he died in 1979.
Yokoi made a trap from wild reeds for catching eels. He also dug himself an underground shelter, supported by strong bamboo canes.
"He was an extremely resourceful man," Hatashin says.
Keeping himself busy also kept him from thinking too much about his predicament, or his family back home, his nephew said.
Return to Guam Yokoi's own memoirs of his time in hiding reveal his desperation not to give up hope, especially in the last eight years when he was totally alone - his last two surviving companions died in floods in 1964.
Turning his thoughts to his ageing mother back home, he at one point wrote: "It was pointless to cause my heart pain by dwelling on such things."
And of another occasion, when he was desperately sick in the jungle, he wrote: "No! I cannot die here. I cannot expose my corpse to the enemy. I must go back to my hole to die. I have so far managed to survive but all is coming to nothing now."
Two weeks after his discovery in the jungle, Yokoi returned home to Japan to a hero's welcome.
He was besieged by the media, interviewed on radio and television, and was regularly invited to speak at universities and in schools across the country.
Find Out More
- Omi Hatashin was interviewed for the BBC World Service programme Witness
- Witness airs every weekday, and tells history through the eyes of the people who were there
According to Hatashin, his uncle grew increasingly nostalgic about the past as he grew older, and before his death in 1997 he went back to Guam on several occasions with his wife.
Some of his prize possessions from those years in the jungle, including his eel traps, are still on show in a small museum on the island.