Construction and expression of synthetic pathways to produce (S) or (R)-3-hydroxybutyrate (3HB) as enantiomerically-pure products by genetically engineering cyanobacterium Synechocystis sp. PCC 6803. Under optimized growth conditions, the pathway employing phaA and phaB from R. eutropha was the most effective, producing up to 533.4±5.5 mg/l (R)-3HB after 21 days photosynthetic cultivation. For the first time, the feasibility and high efficiency of producing 3HB using solar energy and CO2 as sole energy and carbon sources by engineered cyanobacteria is demonstrated.