In this Letter, we demonstrate for the first time (to our best knowledge) stamp printing of silicon nanomembrane (SiNM)-based in-plane photonic devices onto a flexible substrate using a modified transfer printing method that utilizes a suspended configuration, which can adjust the adhesion between the released SiNM and the "handle" silicon wafer. With this method, 230 nm thick, 30 μm wide, and up to 5.7 cm long SiNM-based waveguides are transferred to flexible Kapton films with >90% transfer yield. The propagation loss of the transferred waveguides is measured to be ~1.1 dB/cm. Scalability of this approach to transfer intricate structures, such as photonic crystal waveguides and multimode interference couplers with a minimum feature size of 200 nm and 2 μm, respectively, is also demonstrated.