A sustainable carbon ink highly suitable for 3D extrusion printing for well‐defined, flexible in‐plane all‐solid‐state micro‐supercapacitors (MSC) is presented. By using ethylcellulose as binder and cyrene as solvent the ink contains only non‐toxic and biodegradable ingredients and is therefore a great alternative to inks typically composed of binders like PTFE or PVDF and toxic solvents like NMP or DMF. The MSCs show high areal capacitances of up to 13.41 mF cm‐2 and 35.10 F cm‐3 for 5 mV s‐1 as well as excellent cycle stability even when mechanically stressed. Various suspension additives are explored for tuning the rheological behavior, conductivity, and electrochemical performance of MSCs.