The device, believed to be a relic from World War Two, was found in Avenham Park in Preston at about 15:00 GMT on Friday.
Lancashire Police said the Army's bomb disposal unit had been called and was planning to destroy the bomb, which could have been a danger to the public.
Police cordons were put in place and people were urged to avoid the area.
It is the second time in under a year a wartime bomb has been discovered in the park.
Customers at a nearby cafe were evacuated and a controlled explosion was carried out after a previous discovery in April 2015.
A man with a metal detector discovered an unexploded wartime bomb in a Lancashire park.