The present invention relates to a foldable rainwater delay device (1) for minimising a flood-peak risk caused by a cloud burst in an area having a hard face (40), the foldable rainwater device (1) being configured to be connected with a rainwater distribution conduit (21), such as a drainpipe, configured to lead rainwater away from the hard face, comprising a foldable closed bag (2, 2') made of a flexible material and having a first inlet (3) and a first outlet (4) and an internal volume (V1), a hose (5) having a first end (6) and a second end (7), the first end of the hose being connectable to the first inlet (3), a bypass valve unit (8) connectable to the rainwater distribution conduit (21) and to the second end (7) of the hose, and an outlet valve unit (10) arranged in the first outlet (4).