According to the Southern Regional Hydrometeorological Center, the depression in southeastern waters near the Hoang Sa (Paracel) archipelago is expected to intensify into a storm on Friday.
Combined with southwest winds, it will bring widespread downpours to the south, including HCMC, from Saturday to Tuesday next week.
Moderate to very heavy rains are expected during this period, with total rainfall ranging from 100 to 140 mm.
Thunderstorms and strong gusts may accompany the rain, prompting warnings of localized flooding in low-lying areas.
The rainy season in the Central Highlands and southern Vietnam is forecast to last until early October, with September likely to see the highest rainfall.
The tropical depression also brings heavy rains to Hanoi and northern Vietnam from Friday afternoon. The downpour is expected to last until Sunday and could affect the final rehearsal for the military parade marking the 80th anniversary of National Day on Saturday morning.