The Day River (Vietnamese: Sông Đáy) is a river in Vietnam.

It was formerly known as Hát River (Sông Hát or Hát Giang) or Gián Khẩu River (Sông Gián Khẩu).

The river is a distributary of the Red River, draining into the Gulf of Tonkin.

The river has a length of 240 km[1] and has a drainage basin of more than 7,500 km2,[2] flowing through Hanoi, and the provinces of Hòa Bình, Hà Nam, Ninh Bình and Nam Định.

This article related to a river in Vietnam is a stub.