The 22-year-old Australian will act as cover at Allianz Park during the Rugby World Cup, which takes place between 18 September and 31 October.
Sarries could potentially lose Jamie George and Schalk Brits to England and South Africa duty at the tournament.
"We felt we that we needed some more depth in the hooking position," Sarries director of rugby Mark McCall said.
"Alongside Scott Spurling and Jared Saunders, Dave will provide us with more options.
"He has been training with us since we've returned to pre-season and has fitted into the club very well."
Porecki has had a spell with Super Rugby outfit Waratahs and has been playing for Sydney side Manly Marlins.
Saracens will begin the defence of their Premiership title at home to Sale Sharks on Saturday, 17 October.
Premiership champions Saracens have signed hooker Dave Porecki on a short-term contract.