The Hong Kong Flu was a pandemic outbreak of influenza that began in Hong Kong in 1968 and spread to the United States of America that year. The outbreak ended the following year, in 1969.

Although there was an outbreak of avian influenza in Hong Kong in early 1968, the Hong Kong flu was actually the A type of regular influenza, specifically the first known outbreak of the H3N2 strain (a notation that refers to the configuration of the hemagglutinin and neuraminidase proteins in the virus).

Because of its similarity to the 1957 Asian Flu (which was the H2N2 strain, differing from the Hong Kong flu only in the chemical arrangement of the hemagglutinin protein as a result of antigenic shift) and possibly the subsequent accumulation of related antibodies in the affected population, the Hong Kong flu resulted in much fewer casualties than most pandemics: it is estimated that only 750,000 people died of the virus worldwide (34,000 people in the United States) during the two years (1968-1969) that it was active. It was therefore the least lethal pandemic in the 20th century.