The storm left at least 372 dead in the impoverished Caribbean nation, with the toll likely to rise sharply as rescue workers reach previously inaccessible areas.
Matthew leveled homes, fouled water sources and killed livestock, with victims pleading for help to arrive quickly.
The United Nations has now launched a $120 million flash appeal to cover Haiti's needs for the next three months.
"A massive response is required," UN Secretary-General Ban Ki-moon told reporters.
"Some towns and villages have been almost wiped off the map. These numbers and needs are growing as more affected areas are reached," he said.