The storm made landfall at 3 a.m. and weakened by 7 a.m. over the north central province of Thanh Hoa, with maximum wind speed of 60 kph.
The National Center for Hydro-Meteorological Forecasting has forecasted that in the next hours, the depression will continue moving west and reach the Laos border at 1 p.m., getting weaker.
The center has recorded heavy rains of 120-280 mm in the past 24 hours in areas from Thanh Hoa to the central province of Quang Binh.
It said the northern delta including Hanoi, the northwestern region and the area from Thanh Hoa to nearby Ha Tinh Province will receive rainfall of 50-120 mm on Sunday.
Koguma is the second storm that has entered the East Sea this year. The first storm, Choiwan, made landfall in Taiwan on June 4. It has been forecast that there will be about 12-14 storms and tropical depressions in the East Sea this year, of which five to six storms will directly affect the mainland of Vietnam.