George Keble Hirst (2 March 1909 – 22 January 1994) was an American virologist who was among the first to study the molecular biology and genetics of animal viruses.

Hirst started to work on influenza virus in 1940, only a few years after it had been isolated. He soon discovered that the virus caused red blood cells to clump together. This phenomenon could be used to diagnose influenza, which had previously required growing the virus in ferrets. He invented the haemagglutination assay, a simple method for quantifying viruses, and later the haemagglutination inhibition assay, which measures virus-specific antibodies in serum. In 1942, he discovered the neuraminadase enzyme, showing for the first time that viruses could contain enzymes. Neuraminidase is the target of the neuraminidase inhibitor class of antiviral drugs, including oseltamivir and zanamivir. In 1962, he was also the first to propose the then-revolutionary idea that virus genomes can consist of discontinuous segments.

He co-founded Virology in 1955, the first English-language journal to focus on viruses, and directed the Public Health Research Institute in New York City for nearly 25 years (1956–81).