The US Post Office has unveiled a stamp to memorialise assassinated gay politician Harvey Milk.

According to reports, the stamp will be available to buy from May 22 this year, which is celebrated as Harvey Milk Day.

The stamp will feature a small vertical pride flag in the top left-hand corner, as well as a black-and-white photo of Milk.

Though gay individuals have been featured on US stamps before, it is thought to be the first one ever issued to actively celebrate a gay rights legacy, or to feature a pride flag.

The renowned gay rights campaigner became the first openly gay politician to be elected in America, when he won a seat on the San Francisco Board of Supervisors in January 1978.

He was assassinated in November 1978 along with mayor George Moscone, by fellow supervisor Dan White.

Susan McGowan, director of USPS Office of Stamps and Corporate Licensing, told the Washington Blade that a ceremony is expected to be held to release the stamp, details of which have not yet been confirmed.

A campaign to get a memorial stamp issued for Milk began in 2009.