A sunshade assembly includes two rail seats, a shade member and a guide unit. The shade member has a main portion and two foldable portions connected respectively to left and right sides of the main portion. The guide unit includes two guide members mounted respectively to the rail seats. Each guide member has a guide groove for receiving a portion of a respective one of the foldable portions of the shade member, and a guide surface formed in the guide groove. When the shade member is unwound, the foldable portions pass through the guide grooves and are urged to be folded through guidance of the guide surfaces before entering and being confined in the rail seats.