Mohammad Asghar, from Edinburgh, was tried and condemned for sending letters to government officials claiming to be the Muslim prophet Mohammed.
Polly Truscott, Amnesty's deputy Asia Pacific director, said: "Mohammad Asghar is now facing the gallows simply for writing a series of letters. He does not deserve punishment. No-one should be charged on the basis of this sort of conduct.''
During closed proceedings at the Adiala prison in Rawalpindi, Asghar's defence team argued that the 65-year-old was suffering from mental illness.
They said he had been treated for paranoid schizophrenia in Edinburgh before he returned to Pakistan in 2010.
However, the judges rejected their arguments. Lawyers said they will appeal against the conviction, but remain worried about Asghar's condition and safety.
The British High Commission in Islamabad said it was aware of Asghar's case and provided assistance to him.
A Scottish Government spokesman said: "We strongly opposes the death penalty in all circumstances. As a minimum we would urge the Pakistani authorities to abide by the moratorium they have on the death penalty. Our thoughts are with Mr Asghar's family at this difficult time."