The online recruitment website replaces printing company OKI as primary sponsor from the start of the next season.
In addition to shirt sponsorship, Jobsite branding will appear across the Portsmouth FC website, on match day programmes and tickets and on all official club merchandise.
The sponsorship package also includes the renaming of the Fratton end of the ground to become the Jobsite Stand.
Keith Potts, chief executive of Jobsite, says the sponsorship will further raise recognition of the brand.
“Becoming the main club sponsor for Portsmouth FC will allow us to reach a huge number of our target demographic”, he says.
Earlier this week, Sulaiman Al Fahim (pictured) was appointed chairman of Portsmouth after current owner Alexander Gaydamak reached an agreement “in principle” to sell the club to Al Fahim Asia Associates.