A big tree with diameter of one meter on Nguyen Van Linh Street in Binh Chanh District to the west of the city was uprooted and fell on a man passing by, knocking him unconscious.
The man, whose name has not been revealed, was hospitalized, but succumbed to his injuries later.
Storm Usagi weakened into tropical depression Sunday afternoon, but HCMC suffered severe flooding and saw big trees uprooted.
Heavy downpours that began Sunday morning lasted several hours and inundated many streets in Saigon.
Nguyen Huu Canh, a flood-prone street in Binh Thanh District, Le Van Viet in District 9, Quoc Huong in District 2 and Huynh Tan Phat in District 7 were submerged under 0.5 meters of water after it began raining heavily Sunday morning.
"I think the storm has weakened and did not affect Saigon much, but the rain was still terrible," Le Thi Binh, a woman living in District 7 told VnExpress as she walked her bike on Huynh Tan Phat Street.
Experts have blamed rapid urbanization for the frequent flooding and other problems facing the city.
Usagi, the ninth storm of the year to hit Vietnam, devolved into a tropical depression near Sunday noon after making landfall in the Vung Tau beach town in Ba Ria-Vung Tau Province.
As of 1 p.m. on Sunday, the tropical depression was moving along the coast from the south central Binh Thuan Province to the southern Ben Tre Province with maxium wind speeds of 60 kilometers per hour. It is expected to continue moving further inland before forming a low-pressure zone over the Cambodia border area at 1 a.m. Monday.
Southern localities including Ba Ria-Vung Tau, Long An, Tien Giang, Ben Tre, Tra Vinh and Ho Chi Minh City will continue to see heavy rainfall until Sunday night at least, Le Dinh Quyet from the Southern Hydrometeorological Center told VnExpress.
As of Sunday evening, rains triggered by Usagi had uprooted around 100 trees in Vung Tau and flooded many places in the central provinces of Ninh Thuan and Khanh Hoa. A part of the North-South Railway line in Ninh Thuan has been damaged by erosion, which has also caused landslides that have paralyzed traffic to Khanh Hoa's Cam Ranh International Airport.
Below is footage of strong winds in HCMC's coastal district Can Gio, on Sunday afternoon.