Programmable 3D cell assembly under physiological pH conditions is achieved using electrodeposited stimuli-responsive alginate gels in a microfluidic device, with parallel sidewall electrodes enabling direct observation of the cell assembly. Electrically triggered assembly and subsequent viability of mammalian cells is demonstrated, along with spatially programmable, multi-address assembly of different strains of E. coli cells. Our approach enables in vitro study of dynamic cellular and inter-cellular processes, from cell growth and stimulus/response to inter-colony and inter-species signaling.