The 24-year-old Iran international signed a five-year contract and becomes the eighth new arrival of the summer at the Amex Stadium.
"I'm really delighted to sign for the club. I was really looking forward to being here, seeing the facilities and the people here,” Jahanbakhsh said.
"It's been a long process to get here, with some busy days deciding where to go, but I'm finally here and very happy about my decision.
"I had the opportunity to come here three years ago, and although it was the right place for me, it wasn't the right moment for me to move to England.
"I've decided to come here after three years and I'm looking forward to starting training and seeing my teammates," he concluded.