Described is a kit for constructing retaining walls including multiple interconnecting units. Various units are provided including primary retaining wall units, end units, and corner units. Each unit is square or rectangular in shape and includes an interior volume, a lip on an upper end thereof, and a protrusion on a lower end thereof. The units can be stacked on top of one another and can be secured by engagement of the lip of a first unit with the protrusion of a second unit. The units can also be arranged side-by-side and secured to one another using securement posts and fasteners. The units include one or more exterior surfaces having a finished appearance that resembles wood, brick, or stone, among others. Further, each unit includes one or more drainage apertures thereon. In this way, the present invention provides a versatile system for constructing a retaining wall of a desired configuration.