The death-row convict is Saidur Rahman Bachhu who had confessed of his crimes.
Judge Jayasri Samaddar of the third special tribunal for prevention of children and women repression passed the order.
In addition, the court also fined the lone accused Bachchu Tk 1 lakh as compensation for the bereaved family.
Bachchu is now on the run after he had secured bail in the case.
Saidur Rahman Bachchu, proprietor of Sonali Travels, had said he developed a relationship with the girl over the phone and took her to his office on the 12th floor of Nahar Plaza on May 31, 2012. Later, he killed her and chopped into pieces to conceal the body.