Abstract A fully 3D-printed polymeric heat pipe with high flexibility and low cost was demonstrated in this study. This wickless gravity-assisted heat pipe was fabricated using a commercial stereolithography 3D printer. The interconnected pocket array was designed to reduce the wall thickness to ∼0.1mm. The post-cured heat pipe can be flexed and twisted without tearing or permanent deformation. Experimental studies were conducted to characterize the performance of the heat pipe in vertical and 90° flexed configurations. In addition, visualization based on a high-speed camera was applied to study the boiling process inside the heat pipe. By charging with a compatible dielectric fluid HFE-7100, the present heat pipe achieved 18.6 W heat dissipation over a hot spot with an area of 25×25 mm2. The result showed an effective thermal conductivity of up to 102.7 W/(m·K) when placed vertically. The bubble dynamics and heat transfer data suggested little difference in performance between the vertical and flexed configuration after the startup of the heat pipe at 5.4 W, owing to a high charging mass. Last, a comparison of the present study and other reported fully polymeric flexible heat pipes was made, and future optimization of the heat pipe performance was discussed.