Saeed Malekpour, a 35-year-old Iranian who has permanent residency in Canada, was arrested in 2008 after returning to Iran to visit his ailing father.
He was sentenced to death in December 2010 on a number of charges, including "designing and moderating adult-content websites" and "agitating against the regime."
The letter, published February 10 on the opposition website "Rahesabz" (rahesabz.net), was signed by hundreds of current or former students of Malekpour's alma mater, Iran's Sharif University.
Other signatories included university professors, students, and activists from inside Iran and abroad.
The letter condemns the death sentence as "illegal and inhumane" and describes the charges against Malekpour as absolutely baseless.
The letter also condemns authorities for depriving Malekpour of access to his lawyer and keeping him in solitary confinement.
Malekpour's wife, Zohreh Eftekhari, told Radio Farda on February 10 that she believes he was tortured in prison into making a false confession.
"I met Saeed about three months after he was detained in Evin prison, when he told me about his confessions that were filmed for 25 hours," Eftekhari said. "I was only able to meet him for a half hour, during which I could tell the kind of extreme trauma he underwent for providing the confessions."
The letter also demands that all charges against Malekpour be reviewed in an open court in the presence of a jury.