The pilot ejected before the crash and was taken to the Queen's Medical Centre in Nottingham by air ambulance, a police spokeswoman said. There was no word about the pilot's condition.
No other casualties have been reported.
The Ministry of Defense said the plane crash-landed near the RAF Cottesmore airbase, 100 miles (160 kilometers) north of London, at about 1:50 p.m. (12:50 GMT).
"We can confirm that just before 1350 today a Harrier GR7 aircraft from RAF Cottesmore came down on open farmland near the village of Ashwell," a spokesman said.
"It is too early to say what might have caused the incident. However, a board of inquiry is being convened."
The local fire and rescue service said 10 rescue vehicles were sent to the scene of the crash.
