The Sierra Leone international, known as Medo, joins for an undisclosed fee and has signed a three-and-a-half year contract.
The 25-year-old has enjoyed a successful spell with Partizan, winning two league titles and a Serbian Cup and playing in the Champions League.
Before moving to Serbia Medo played for Finnish sides Helsingin Jalkapalloklubi (HJK) and Kuopion Palloseura (KuPS), winning one league title and two domestic cups over a four-year period.
Boss Dougie Freedman told the club's website: "I've known about Medo for a while now. Aki Riihilahti, who is a good friend of mine from when we were Crystal Palace, played with him in Finland.
"I've seen a lot of him and never really thought I'd have any chance of working with him. From Finland he went over to Partizan Belgrade and has certainly become a better player over the last three or four years.
"It's the right deal for the club. He has the heart of a lion, the hunger, the desire and the right mind-set - he wants to come here and do well and I look forward to working with him."