Microalgal starch, a potential feedstock for renewable fuels and chemicals, is usually induced under stress, which hinders the simultaneous enhancement of starch content and yield. This study applied gamma-aminobutyric acid (GABA) to modulate cell metabolism for improving photosynthetic starch production in microalga Tetraselmis subcordiformis under nitrogen limitation (+/- N) and depletion (-N). Under +/- N, 2.5 mM GABA abated nitrogen metabolism but enhanced starch content by 27% with an uncompromised starch yield due to the moderate inhibition of cell growth and elevated photoprotection. Under -N, 10 mM GABA entitled improved stress tolerance and enhanced starch content by 23.4% with a simultaneous 28.6% promotion of starch yield. The maximum starch content and yield under this condition reached 69.0% DW and 1.84 g L-1, respectively, which represented the highest starch levels in this microalga and exceeded most of the starch production performance among the microalgae reported. GABA addition was promising to improve microalgal starch production.