The Munster scrum-half, who has won 98 caps, will be registered to play Premiership and Heineken Cup rugby with Saracens.
Saracens' move for Stringer follows injuries suffered by their England number nine Richard Wigglesworth and South African Neil de Kock.
"We are fortunate to have a pair of outstanding young scrum-halves in Ben Spencer and Luke Baldwin, but the injuries to Richard Wigglesworth and Neil de Kock have left us exposed in this pivotal position," Saracens rugby director Mark McCall said.
"We are delighted to have secured the services of Peter Stringer, an experienced player of proven quality."
Saracens are next in action on Saturday when they resume their Heineken Cup campaign by tackling the Ospreys at Wembley.