The Moroccan becomes the Saints' fourth signing of the summer following the arrivals of Adam Hammill, Gregory Vignal and Alan Bennett.
"I'm very pleased that we've signed Yousseff as he's a player that I've admired for a long time," said Burley.
"He's a good passer of the ball and a decent playmaker in midfield, he's not bad at set-plays either, so there are a lot of aspects to Safri's game."