The growing demand for rechargeable lithium‐ion batteries (LIBs) with higher capacity in customized geometries underscores the need for new battery materials, architectures, and assembly strategies. Here, the design, fabrication, and electrochemical performance of fully 3D printed LIBs composed of thick semisolid electrodes that exhibit high areal capacity are reported. Specifically, semisolid cathode and anode inks, as well as UV curable packaging and separator inks for direct writing of LIBs in arbitrary geometries are created. These fully 3D printed and packaged LIBs, which are encased between two glassy carbon current collectors, deliver an areal capacity of 4.45 mAh cm−2 at a current density of 0.14 mA cm−2, which is equivalent to 17.3 Ah L−1. The ability to produce high‐performance LIBs in customized form factors opens new avenues for integrating batteries directly within 3D printed objects. Fully 3D‐printed and packaged lithium‐ion batteries (LIBs) composed of thick, semisolid biphasic electrodes are created in customized form factors. These 3D‐printed, rechargeable LIBs exhibit high areal energy and power densities.