US lock Smith returns to Saracens from NFL
Curtis Brinkley and Antoine Cason (R) of the San Diego Chargers take down Hayden Smith of the New York Jets at MetLife Stadium on December 23, 2012 in East Rutherford, New Jersey
As a tight-end, Smith made six appearances in the US National Football League but is returning to the London club where he played between 2008-12.
"It's fantastic to be back," said the lock capped 18 times by the United States.
"I learned a huge amount in the NFL, various elements that will help me become a better rugby player, but there is nothing like the Saracens environment and I am now completely focused on working hard and helping the squad achieve all our goals."
Saracens director of rugby Mark McCall added: "Hayden is a phenomenal athlete with huge potential, and it's great to have him back in the fold."