An under-cover support structure comprising: a dash panel provided in a front part of a vehicle cabin; a dash insulator attached to and extending along a cabin-side surface of the dash panel; an instrument panel mounted on the dash insulator in an inner side of the vehicle cabin; and an under cover member bridged between the cabin-side surface of the dash panel and a forward edge of a lower edge portion of the instrument panel, and covering a lower side of the instrument panel, the under cover member having a press-fit end portion on a side of the dash insulator, wherein the dash insulator is provided with a supporting portion which is elastically deformable by the press-fit end portion of the under cover member being pressed against the supporting portion.