The researchers constructed a bacterium's "genetic software" and transplanted it into a host cell.
The resulting microbe then looked and behaved like the species "dictated" by the synthetic DNA.
The advance, published in Science, has been hailed as a scientific landmark, but critics say there are dangers posed by synthetic organisms.
Some also suggest that the potential benefits of the technology have been over-stated.
But the researchers hope eventually to design bacterial cells that will produce medicines and fuels and even absorb greenhouse gases.
The team was led by Dr Craig Venter of the J Craig Venter Institute (JCVI) in Maryland and California.