Bonhams said the portrait is rare because it appears dated to the final years of Keats' life. He died at age 25 in 1821 and most portraits of him were done posthumous, based off a painting of him seated. This portrait shows Keats standing against a blue sky background, The Daily Telegraph said.
It is further authenticated because the image was used on the cover of a Stanford University Press anthology of Keats poems, published in 1933.
In back of the frame is a lock of dark blond hair.
Bonhams said the painting could be worth as much as $23,000.
The artist is unknown but researchers said they suspect it may have been Charles Hayter, who produced miniature portraits akin to this one around that time, Bonhams said.
It was thought to have been purchased by the recently deceased owner in the 1950s in San Francisco but how it got to the United States is unknown.
The Keats portrait will go on the bloc alongside a group of five pencil drawings by the artist John Constable. That collection could be worth "a six figure sum," Bonhams said.