Soft and stretchable electronic materials have a number of unique applications, not least within sensors for monitoring human health. Through development of appropriate inks, micro-extrusion 3D printing offers an appealing route for integrating soft electronic materials within wearable garments. Toward this objective, here a series of conductive inks based on soft thermoplastic styrene–ethylene–butylene–styrene elastomers combined with silver micro-flakes, carbon black nanoparticles, or poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer additives, is developed. Their electrical and mechanical properties are systematically compared and found to be highly dependent on additive amount and type. Thus, while silver composites offer the highest conductivity, their stretchability is far inferior to carbon black composites, which can maintain conductivity beyond 400% strain. The PEDOT composites are the least conductive and stretchable but display unique properties due to their propensity for ionic conductivity. To integrate these inks, as well as insulating counterparts, into functional designs, a multi-material micro-extrusion 3D printing routine for direct deposition onto stretchable, elastic fabrics is established. As demonstration, prototypes are produced for sensing common health markers including strain, physiological temperatures, and electrocardiograms. Collectively, this work demonstrates multi-material 3D printing of soft styrene–ethylene–butylene–styrene elastomer composites as a versatile method for fabricating soft bio-sensors.