The Maltese defender, 25, was at Ebbsfleet United last season but left on the expiration of his contract.
Danny Kedwell and Andy Drury have also followed Magri to the Hawks who blew away the Monday blues with a triple whammy of good news.
A commanding presence in the Fleet's side last season, Paul Doswell believes Magri can be vital to his side's promotion aspirations.
“It’s a great signing for both parties," Doswell told the club's website. “From our perspective we’ve signed a 25-year-old Maltese international who I thought was one of the best centre-halves in The National League last year and consistently one of the best over the last few seasons.”
“When we knew he hadn’t been offered a new contract we moved quickly knowing that Sam was a Portsmouth lad," he added.
Elsewhere, Danny Kedwell joined Magri in agreeing terms at the Hawks.
The 35-year-old has a keen eye for a goal and has previously helped the Fleet into the National League.
"He will be captain of this football club and someone who will bring leadership, enthusiasm and ability," Doswell added after confirming an incredible double swoop.