Amateur photographer Martin Le-May captured the incredible image after he heard a "distressed squawking" while wandering through Hornchurch Country Park, in east London, with his wife.
The photograph has now gone viral online after thousands of users shared the "one-in-a-million" snap on social media.
Mr Le-May tweeted the picture on Monday, adding: "Took this picture earlier today.
"Heard woodpecker in distress, then flew up with Weasel on back."
It is believed that the weasel had been attacking the green woodpecker after it took flight in a desperate bid to escape.
Witnesses report both animals escaped the tussle unscathed after the woodpecker was able to shake off the unwelcome furry hitchhiker.
Richard James, a wildlife advisor for the Royal Society for the Protection of Bird (RSPB), described the image as "a one-in-a-million shot."
He said: "These are an incredible set of images.
"Weasels are ferocious predators and often attack prey much larger than themselves.
"In this case the weasel appears to have targeted the woodpecker due to the fact the green woodpeckers often spend a lot of on the ground eating ants so is very susceptible to attack.
"If this had been a weaker bird I'm sure the attack would have been successful but this woodpecker was strong enough to take flight with the weasel on his back."
Describing it as a "fortunate escape", Mr James added: "Green woodpeckers are relatively common but to see it in a mid-air battle like this is incredibly unusual."
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