This paper presents a novel digital prefabrication concept for the ribbed slabs, wherein post-tensioned 3D printed concrete (3DPC) serves as a lightweight stay-in-place formwork. The formwork is designed to withstand the dead loads from its own weight and the cast concrete topping, along with live loads from construction equipment. The proposed concept simplifies the construction process for reinforced concrete (RC) slabs by eliminating the need of temporary formwork, temporary shoring or scaffolding typically required in traditional construction. To evaluate the feasibility of the concept, design calculations were performed for a ribbed slab formwork with 30 mm thickness. The results showed that the proposed concept is feasible for span lengths of 4 to 6m for various eccentricities of the post-tensioning tendons. The printing feasibility of this slender formwork design was evaluated by printing it to a height of 2 m. The paper concludes with a discussion on the design implications of the corrugated surface of 3DPC compared to flat surface of conventional concrete. For this purpose, a detailed analysis was conducted to assess the stress concentration arising from the corrugated geometry of 3DPC and its consequent effects on the cracking loads.