The Frenchman is eligible to play against holders Liverpool in round of 16 of the UEFA Champions League which was a decisive factor in his decision.
"This is a super club. To be able to play in the Champions League is fantastic and that's why I am here," Robert was quoted as saying by BBC on Wednesday.
"They showed a lot of interest in me. I am very happy to be here. Knowing a coach like Koeman really wanted me in his side was enough to convince me it was the right thing to do," he added.
Robert cut his short loan spell at Portsmouth in order to seal a move to Benfica after a several bust-ups with Newcastle coach Graeme Souness as well as his failure to establish himself at Portsmouth.
The 30-year-old said he hopes to be included in France coach Raymond Domenech's plans for the World Cup finals in Germany.
"The World Cup might seem a long way off for a player like me, but I haven't given up hope," he said.
"I still think I could do a job for my country and I'd love another chance to prove it," he added.