Saracens rugby director Alan Gaffney said: "Francisco is a player we have been keeping an eye on for some time, and we are delighted he has joined Saracens.
"He adds strength and balance to the squad and is a very powerful and strong runner. He is recognised as being a full-back, but he can also play on the wing or even at outside centre.
"He is an experienced international and played in the summer Tests against Ireland and Italy. He is unlucky not to be going to the World Cup."