The 28-year-old, identified as Evans Muncie, was found under the remains of the Croix Bossal market where he sold rice. He had not been seen since the 7.0 magnitude earthquake levelled the city on January 12, killing over 200,000 people.
Mr. Muncie was discovered by people who had been digging at the marketplace. He was taken to an American field hospital in Port-au-Prince where he was treated for severe dehydration and malnutrition.
Doctors said Mr. Muncie was much disorientated and at times thought he was still under the rubble while he was being treated at the tent hospital.
It could not be confirmed exactly how long Mr Muncie was trapped but his family said he had been missing since January 12.
Dr Mike Connelly told CNN, the American news network: "He was emaciated. He hadn't had anything in quite some time. He had open wounds that were festering on both of his feet."
According to CNN, Mr. Muncie told doctors that somebody in a white coat had occasionally brought him water; however it is not known whether he had been hallucinating.
He also said he could hear bulldozers around him working to demolish damaged buildings while he lay underneath the rubble near by.
Dr Connelly said Mr. Muncie must have had access to water to have survived the entire four weeks trapped under a building.
The US network showed photographs of Mr.Muncie being treated by doctors.
Despite the Haitian Government calling for an end to search and rescue operations on January 23, survivors continued to be pulled from the rubble in the following days.
On January 27, 16-year-old Daline Etienne was rescued from beneath the debris of her house; 15 days after the earthquake had forced the building to collapse on top of her.
The rescue of the teenager, who appeared in relatively good health, was deemed a miracle.
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