The States police cordoned off the scene, close to the new Westmount flats, after the discovery was made on Tuesday afternoon.
Following analysis, it is believed the skull is from a body buried at the former garrison and strangers’ cemeteries which opened at the bottom of Westmount in 1832.
The strangers’ cemetery was reserved for people who had no real connection to the parish.
The site was cleared in the 1970s to make way for flats and the bodies were exhumed and moved to the modern-day Westmount Cemetery at the top of the hill.
The skull was from one of several bodies which were missed during the clearance of the area.
Olga Finch, curator of archaeology at Jersey Heritage, who has examined the skull, said: ‘We know that a cemetery once stood on the site and so that helps put it into context for the police.
‘A few other bones were also found in the area about 18 months ago.
‘It is definitely an adult skull and it is clearly historical. I believe it has probably been dug up before and then deposited where it was found this week. I don’t believe it was buried in that exact spot.’
The skull has now been taken to the mortuary and the Viscount’s Department have been informed. It will undergo further examinations before being buried.