"It's nice to be here," Senegal international Diop told the Pompey website. "It's a good move for me and a good move for the club.
"It was an easy decision to make and I am very happy."
Diop added: "Three years ago was the first time I'd come to England and I didn't understand English football very well.
"Now I understand it very well and I'm happy that I'm at Portsmouth.
"It's different to Fulham. The fans here are hot for 90 minutes."