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.

Guards at the site saw the pair pick up the items from the ground, the judge at a court in Krakow was told.
The pupils, from the independent Perse School in Cambridge, say they picked them up to show their guide.
They deny stealing artefacts of special cultural importance during a school visit in June 2015.
The court in Krakow heard the items were found on the students by police.
The pupils were in an area of the Nazi death camp where the belongings of Jews who were sent to the gas chambers were kept, the court was told.
The court heard police also found a fragments of a spoon and glass on the pair, who were 17 at the time.
Initially they pleaded guilty to the charge and accepted a fine, but once back in the UK they changed their pleas.
If found guilty, they face prison sentences of up to 10 years.
The Nazis murdered 1.1 million people at Auschwitz in occupied Poland during World War Two.
The trial continues.
A man with a metal detector discovered an unexploded wartime bomb in a Lancashire park.
[NEXT_CONCEPT]
Two British teenagers stole items including buttons and a rusted hair clipper from the Auschwitz death camp, a court in Poland has heard.